Category: Sizing & How-to

For a 3-day water backup, FEMA recommends 1 US gallon (3.78 litres) per person per day as a survival minimum — but this covers only drinking and basic sanitation with no cooking, bathing, or toilet flushing. For functional 3-day backup that maintains normal household operations, the figure is 50–150 litres per person per day, meaning a family of four needs 600–1,800 litres in storage. This article explains the range, how to set the right number for your situation, and what infrastructure you need to store it reliably. If you’re specifically preparing to FEMA’s framework, see our companion guide on emergency water storage for US households: FEMA guidelines.

The Quick Answer

Three-day water storage targets depend on use level. FEMA’s minimum (1 gallon/person/day, per FEMA Ready.gov guidelines) is a survival threshold — it assumes no bathing, no toilet flushing, and minimal cooking. A commonly cited minimum for basic humanitarian needs (drinking, food preparation, and basic hygiene) is 15 litres/person/day. Normal household operation requires 50–150 litres/person/day depending on climate and habits.

Household size	FEMA minimum (72 hrs)	Basic humanitarian minimum (72 hrs)	Normal household use (72 hrs)
1 person	11.4 L (3 gal)	45 L	150–450 L
2 people	22.7 L (6 gal)	90 L	300–900 L
4 people	45.4 L (12 gal)	180 L	600–1,800 L
6 people	68.1 L (18 gal)	270 L	900–2,700 L
4 people + 1 infant	45.4 L + 15 L formula	220 L	700–2,000 L

Skip the math: Use the 72-Hour Water Supply Calculator to get a 3-day storage target based on your household size, climate, and intended use level — survival minimum through full household operation.

How the Calculation Works

Step 1 — Establish your daily consumption baseline. The most accurate figure comes from your water meter. Read it at the same time on two days with normal usage. Divide the total by 2 (or the number of days) and by the number of occupants. This is your actual per-capita daily consumption. If you’d rather not rely on a meter reading, our guide on sizing a water tank for daily household use breaks daily consumption down activity by activity.

Step 2 — Apply an emergency reduction factor. During a supply interruption, toilet flushing (which accounts for 25–30% of normal household use, per US EPA WaterSense data) can be reduced or eliminated. Outdoor water use stops entirely. Showers get shorter or are replaced with sponge bathing. For a household actively conserving, consumption typically drops to 30–50% of normal levels — a reduction of 50–70%.

Step 3 — Worked example. Family of 4, normal consumption 180 litres per day per person = 720 litres/day total. Emergency mode consumption (60% reduction, i.e. 40% of normal retained) = 720 × 0.4 = 288 litres/day. Over 3 days: 288 × 3 = 864 litres. Round to the next standard tank size: 1,000 litres

Step 4 — Add a safety margin. FEMA recommends building in reserve beyond the 72-hour target because emergencies rarely resolve exactly on schedule. A 20% buffer is standard: 864 × 1.2 = 1,037 litres. A 1,000-litre tank is the minimum; 1,200–1,500 litres provides adequate margin.

Key Variables That Change the 3-Day Storage Requirement

Climate and ambient temperature. In hot climates (above 35°C), per-person drinking water requirements increase from around 2 litres/day to 4–6 litres/day, and physical labour outdoors in heat raises this further. A 3-day minimum for active adults in a hot climate must account for this — the FEMA gallon-per-day figure was developed for temperate conditions.

Medical and sanitary requirements. Dialysis patients typically require 120–150 litres of clean water per session, 3 sessions per week. A 3-day backup for a household with a dialysis patient requires this volume on top of household consumption. Similarly, households with infants need water for formula preparation and sterilisation — typically an additional 5–8 litres per day per infant.

Whether you have flush toilets. Standard flush toilets use 6–13 litres per flush (US EPA data — older toilets at the higher end, modern standard toilets nearer 6 L). At 5 flushes per person per day, a family of 4 uses 120–260 litres just for toilets. If supply is cut and mains pressure is lost, toilet flushing requires manual tank filling. Many households do not account for this — they calculate drinking and cooking water only, then find their storage depleted in 12–18 hours by toilet use alone.

Whether the storage is pre-treated. Stored water must be treated for it to remain safe across a 72-hour period, particularly in warm climates. Untreated water from a tap stored in a clean container is safe for 6–12 hours at room temperature; treated water with 0.2–0.5 mg/L free chlorine remains safe for 6–12 months in a sealed container. If your 3-day storage is not chemically treated, it needs to be rotated continuously or used immediately on a supply interruption. Our guide on how chlorination of water tanks actually works explains how to dose and maintain this residual correctly, and why water tastes or smells bad after storage covers the warning signs that stored water has gone past safe use.

Pets. A medium-sized dog requires 30–50 ml per kg of body weight per day (approximately 1 litre/day for a 25 kg dog). A household with two dogs and a family of four needs to add 2+ litres per day — small in absolute terms but worth including in a precise calculation.

Common Mistakes When Planning 3-Day Water Backup

Calculating drinking water only and ignoring sanitation. The FEMA 1 gallon/day figure is explicitly a survival minimum for drinking only. People planning a ‘3-day emergency supply’ using this number are typically unprepared for toilet flushing, cooking, wound cleaning, and medication preparation. Real 72-hour preparedness for normal function requires 10–20× the FEMA minimum per person.

Storing water in food-grade containers that are not food-grade. HDPE plastic containers rated food-grade (marked with recycling code 2) are suitable for water storage. Containers that previously held bleach or chemicals, or are made from non-food-grade plastics, leach compounds into stored water. Standard garden water barrels are often not food-grade. Containers must also be opaque — clear or translucent containers allow algae growth in any light.

Not accounting for rotation. Water stored without treatment and without rotation becomes unsafe. Even treated water in a non-sealed container should be replaced every 6 months. Many households set up 3-day storage and forget it — inspecting it years later to find sediment, bacterial growth, or degraded container integrity.

Ignoring access during power outages. If your backup storage depends on an electric pump to access it — for example, underground cisterns or pressurised systems — a power outage simultaneously cuts water supply and pump operation. Gravity-fed rooftop tanks or hand-pump accessible containers are the only reliable 3-day backup for power-cut scenarios. Our guide on how gravity-feed water systems work explains why these setups keep functioning when pumps don’t.

Related Calculators You Might Need

The Emergency Water Storage Calculator lets you customise your target by intended use level and household composition. If you want to plan beyond 72 hours, the Water Stockpile Duration Calculator tells you how long any given stored volume will last at your consumption rate. For households preparing for specific events, the Hurricane and Disaster Water Prep Calculator applies scenario-specific guidance. And if you are evaluating the ongoing cost of maintaining emergency storage versus other supply options, the Water Delivery vs Tank Cost Calculator puts the numbers into context.

Frequently Asked Questions

How much water does a family of 4 need for 3 days?

At FEMA’s survival minimum of 1 US gallon (3.78 L) per person per day: 45.4 litres total — covering drinking only. For normal household operation with active emergency conservation: roughly 650–1,050 litres, based on a normal daily use of 150–180 L/person reduced to 30–50% during conservation. The correct figure depends entirely on how you define ‘backup’ — survival hydration only or maintained household function. Use the 72-Hour Water Supply Calculator to calculate for your specific household.

What size water tank do I need for a 72-hour emergency supply?

For most households aiming at functional (not survival) backup, the practical tank size is 200–300 litres per person for 72 hours. A family of four therefore needs an 800–1,200-litre tank dedicated to emergency use. This can be a standalone polyethylene tank with a sealed lid, a gravity-fed overhead unit, or a purpose-built underground cistern — depending on access and structural constraints.

Can I use my existing water tank as a 3-day backup?

Yes, if it meets three criteria: the stored volume is adequate for your 72-hour target, the water is treated and rotated on schedule, and you can access it independently of mains pressure and electric pumps. Many household overhead tanks already hold 500–2,000 litres — check whether the usable volume (subtracting the dead zone below the outlet) meets your 3-day requirement before purchasing additional storage.

How long does stored water last before it goes bad?

Treated tap water in a sealed, food-grade, opaque container remains safe for 6–12 months. Water stored in open or semi-open containers should be replaced every 48–72 hours. Commercially bottled water has a stated shelf life of 12–24 months, though this reflects container integrity, not water chemistry — properly stored water does not expire the way food does; the container degrades.

Is 1 gallon of water per day per person enough for 3 days?

For survival — yes. FEMA’s 1 gallon/person/day is sufficient to prevent dehydration in a temperate climate with minimal activity. It is not sufficient for cooking, hygiene, toilet flushing, or any medical needs. If you are planning for a functional emergency period rather than strict survival, multiply the FEMA figure by 5–15 depending on your specific household requirements.

A commercial building’s water tank is sized from three numbers: how many people use the building, how much each one uses in a day for that type of building, and how long the tank must last without a refill. Multiply them together and add 20%. An office with 300 staff, at 45 litres each, covering a 2-day gap needs 300 × 45 × 2 × 1.2 = 32,400 litres.

Floor area does not come into it, and neither does the figure you would use for a house. An office worker uses about a third of what the same person uses at home, while a hospital bed uses three times as much. Fire-fighting water is a separate volume again, set by the fire code and never part of this sum. This guide gives the formula, the per-person figures by building type, four worked examples and the checks to make before ordering.

The commercial tank sizing formula

Tank size (litres) = Occupants × Litres per occupant per day × Storage days × 1.2
Then add fire-fighting water separately, if your building needs it.

Input	What it means	Where to get it
Occupants	The people, beds or seats that drive water use at the busiest normal time	Headcount, room count, bed count or seating plan (Step 2)
Litres per occupant per day	A daily rate for that type of building	A code table, or better, the building’s own meter (Step 1)
Storage days	How long the tank must supply the building with nothing coming in	Your supply pattern (Step 3)
× 1.2	A 20% buffer	Covers busy days, growth and the water below the outlet

Step 1: Find the daily rate for your building type

Daily rates come from building codes, and the codes do not all agree. The most complete public table is in India’s standard for water supply in buildings, and the second column below shows the rounded planning rates used in our calculator. Use the figure from the code that applies where you build.

Building type	IS 1172 (India), litres per day	Our calculator’s planning rate
Office	45 per person	45 per staff member
Hotel	180 per guest	200 per guest
Hospital, up to 100 beds	340 per bed, including laundry	300 per bed
Hospital, over 100 beds	450 per bed, including laundry	300 per bed
Day school	45 per student	30 per student
Boarding school or hostel	135 per resident	Not listed
Restaurant	70 per seat	40 per meal served
Factory	30 per worker, or 45 where bathrooms are provided	65 per worker
Cinema, concert hall, theatre	15 per seat	Not listed
Retail or shopping centre	Not listed	30 per occupant

Three points about using a table like this:

• Watch the unit. “Per seat” and “per meal served” are different counts, and so are “per bed” and “per guest”. A 60-seat restaurant that serves 200 meals a day gives two different answers depending on which rate you pick. Use the unit that matches the rate.
• These are design figures, not minimums. For comparison, WHO’s emergency guidance for health facilities is 40–60 litres per inpatient per day and 5 litres per outpatient (WHO technical note). That is a survival floor for a field hospital. A permanent hospital with laundry, kitchens and flush toilets is designed for roughly six to ten times as much.
• A meter beats a table. If the building exists, or a similar one does, take a month of metered use and divide by the days it was open. How to read a water meter shows the method, and the Water Meter Reading Calculator does the arithmetic.

Do not use a household figure. The same standard allows 135 litres or more per person per day in homes, three times the office rate, because people bathe, cook and wash clothes at home. Sizing an office on the household figure gives a tank three times too large.

Step 2: Count the right people

• Offices: the number of staff present on a normal busy day, plus regular visitors. If half the workforce is at home on any given day, the registered headcount overstates demand.
• Hotels: rooms × guests per room at full occupancy. A hotel has to cope with a full house.
• Hospitals: beds, not staff. The per-bed rate already allows for the staff and services each bed needs.
• Schools: enrolled students, plus staff.
• Mixed buildings: work out each part at its own rate and add them. Shops on the ground floor, offices above and flats at the top are three separate sums. For the residential part, use the Apartment Water Tank Size Calculator.

Then add anything the per-person rate does not cover. A staff canteen or hotel restaurant is added at the restaurant rate. Cooling towers, laundries taking outside work, landscape irrigation, vehicle washing and process water are all extra, and each needs its own estimate from the equipment supplier or from meter records.

Step 3: Choose the storage period

Supply to the building	Storage to plan for	Why
Continuous mains, rare outages	1 day	Covers a burst main or a day of repair work
Mains for a few hours every day	1–2 days	One missed window should not close the building
Mains on alternate days or irregular	2–3 days	The normal gap, plus one skipped turn
Tanker delivery	Days between deliveries, plus 1–2	Deliveries run late when demand is highest
Hospital or other critical facility	As your health authority requires	Resilience rules are set locally and are stricter than for ordinary buildings

Where supply is intermittent there is a second test: the tank has to refill inside the supply window. Divide a day’s demand by the minutes of supply to get the inflow you need.

Example: an office uses 13,500 L a day and the mains runs for 4 hours each night. It needs 13,500 ÷ 240 = 56 L/min coming in during that window just to break even. If the connection delivers less, no tank size will keep the building supplied.

Check the inflow with the Tank Refill Time Calculator, and see why a water tank takes so long to refill if it falls short.

Step 4: Add the buffer and split the storage

The × 1.2 adds 20%, which is the default in our calculator. It covers unusually busy days, some growth in occupancy and the water below the outlet that the tank cannot deliver.

At commercial volumes, plan the storage as two tanks or two compartments, not one. Either can then be drained for cleaning or repair while the other keeps the building open. Single large tank vs multiple small tanks covers the trade-offs.

Worked examples

Office, 300 staff, intermittent supply

1. Daily demand: 300 × 45 = 13,500 L.
2. Two days of storage: 13,500 × 2 = 27,000 L.
3. Buffer: 27,000 × 1.2 = 32,400 L.
4. Specify: about 35,000 L, as two tanks.

Hotel, 80 rooms with a 60-seat restaurant, reliable supply

1. Guests: 80 rooms × 2 = 160 guests at full occupancy. 160 × 180 = 28,800 L.
2. Restaurant: 60 seats × 70 = 4,200 L.
3. Daily demand: 28,800 + 4,200 = 33,000 L.
4. One day with buffer: 33,000 × 1.2 = 39,600 L. Specify 40,000 L.

Hospital, 120 beds

1. Daily demand: 120 × 450 = 54,000 L (the over-100-bed rate, including laundry).
2. Two days of storage: 54,000 × 2 = 108,000 L.
3. Buffer: 108,000 × 1.2 = 129,600 L. Specify 130,000 L, and confirm the storage period with the health authority.

Day school, 500 students

1. Daily demand: 500 × 45 = 22,500 L.
2. One day with buffer: 22,500 × 1.2 = 27,000 L.

For schools, colleges, hostels and hospitals, the School and Institutional Water Tank Size Calculator has the rates built in.

Office tank size chart

Domestic storage in litres for an office at 45 L per person per day, with the 20% buffer included and fire water excluded.

Staff	1 day	2 days	3 days
100	5,400 L	10,800 L	16,200 L
250	13,500 L	27,000 L	40,500 L
500	27,000 L	54,000 L	81,000 L
1,000	54,000 L	108,000 L	162,000 L

Fire-fighting water is a separate volume

Nothing in the formula above provides water for fire-fighting. That volume is set by the fire code for your building’s height, use and fire systems, and it has to be available in full at all times. The building’s daily use must never be able to draw it down.

Water-supply standards say this themselves. IS 1172 gives no fire quantity and instead directs the designer elsewhere: “For fire demand in buildings refer IS 9668”. Our calculator leaves fire water out for the same reason. The standards that set it include NFPA 13 and NFPA 22 in the United States, BS EN 12845 in the UK and Europe, AS 2419.1 in Australia and the National Building Code in India.

Get the fire volume from the fire engineer or the fire authority, in writing, before you size anything. It depends on the systems installed, and for a sprinklered building it can be larger than the whole of the daily-use storage. Do not estimate it from a rule of thumb, and do not count it as usable water.

Bigger is not safer: avoid stale water

In a house, an oversized tank wastes a little money. In a commercial building it creates a water-quality problem. Water that sits in a tank for days loses the chlorine it arrived with and warms towards room temperature, and those are the conditions bacteria need. The measure to watch is turnover:

Turnover (days) = Tank volume ÷ Daily demand
The 35,000 L office tank above, at 13,500 L a day, turns over every 2.6 days.

Practical ways to keep stored water fresh:

• Size for the demand you have, not for a worst case that may never come. Add a second tank later if occupancy grows.
• Make the water pass through. UK guidance for storage cisterns puts the outlet on the opposite side from the inlet, so fresh water sweeps across the tank instead of leaving a dead corner.
• Connect twin tanks so both are in use, not one working and one standing.
• Keep tanks cool. The same guidance calls for cisterns to be “sited away from heat sources and insulated”.
• Plan for closures. A school over the summer or an office over a long holiday has full tanks and no demand. Lower the level beforehand or flush the system before reopening.

For treatment, see how chlorination of water tanks works and the Chlorine Dosage Calculator. The Safe Water Storage Duration Calculator gives guidance on how long stored water keeps.

Four checks before you order

1. Where will the weight go?

Water weighs one tonne per 1,000 litres. The 32,400 L office tank is 32 tonnes of water, and the hospital example is 130 tonnes. Volumes like these normally go in a ground-level or basement tank, with a pump lifting water to a much smaller roof tank or straight into the building. Overhead vs underground water tanks explains the arrangement. Anything on the roof needs a structural engineer: start with the Rooftop Load Bearing Calculator and how to size a rooftop water tank for a multi-storey building.

2. Will the top floor have pressure?

A roof tank gives about 0.1 bar for each metre of height, so the top floor always gets the least. If the top-floor washrooms sit only 2 or 3 metres below the tank, they will need a booster pump. How gravity feed water systems work has the figures.

3. Can the pipes meet the peak?

Commercial demand comes in bursts: arrival time, breaks, lunch, the end of the day. A tank with the right volume still fails if the pipes cannot deliver the peak flow. Our calculator shows the average flow while the building is open, which is daily demand divided by operating hours; the peak is higher than that. Check the outlet and riser sizes with the Pipe Size and Flow Rate Calculator, and the pump with the Pump Head Pressure Calculator.

4. Can it be inspected and cleaned?

A tank needs a way in, a way to drain it and room to work. UK guidance asks for “a rigid close fitting and securely fixed lid or cover”, a screen on the overflow with mesh no larger than 0.65 mm to keep insects out, and proper support under the base (Water Regs UK). On inspection intervals, Illinois EPA’s guidance for public water systems is a useful benchmark: a storage tank “should be inspected at least every five years, or more often, depending on local conditions” (Illinois EPA). Commercial buildings often fall under stricter local health rules, so check yours.

Mistakes to avoid

• Sizing from floor area. People use water. Square metres do not.
• Using a household rate for an office. It triples the tank.
• Counting fire water as part of the daily storage, or letting daily use draw on it.
• Mixing up units. Per seat is not per meal, and per bed is not per guest.
• Leaving out the kitchen, laundry or cooling towers. Per-person office and hotel rates do not include them.
• Oversizing “to be safe”. Slow turnover is a water-quality risk.
• Buying one tank. It cannot be cleaned without closing the building’s water supply.
• Checking volume but not refill rate. Where supply is intermittent, the inflow decides whether the tank ever fills.

More calculators and guides

• Water Tank Weight Calculator: the full weight of any tank.
• Water Tank Cost Calculator: budget figures by material and size.
• Plastic vs steel vs concrete water tanks and polyethylene vs fibreglass water tanks: choosing the material for large storage.
• How to calculate the right water tank size for your home: the household version of this method.

Frequently asked questions

How do I calculate the water tank size for a commercial building?

Multiply the number of occupants by the daily rate for the building type, then by the days of storage, then by 1.2. For 300 office staff at 45 L each and 2 days of storage: 300 × 45 × 2 × 1.2 = 32,400 L. Add fire-fighting water separately if the fire code requires it.

How much water does an office use per person per day?

India’s IS 1172 uses 45 litres per person per day for offices, and that is the planning rate in our calculator. It covers toilets, hand washing and drinking water. A canteen, showers or cooling towers are extra. Other codes use different figures, so check the one that applies to your project.

Does a commercial water tank need to include fire storage?

Many commercial buildings must store water for fire-fighting, but it is a separate volume from the daily-use storage and is set by the fire code, not by a water-use formula. Ask the fire engineer or the fire authority for the figure, and make sure daily use cannot draw it down.

How many days of water storage does a commercial building need?

Enough to cover the longest normal gap in supply. That is about 1 day on reliable mains and 2 to 3 days where supply is intermittent. Hospitals and other critical buildings follow their health authority’s resilience rules. More is not better: storage beyond what the building uses in a few days sits and goes stale.

How much water does a hotel need per room?

Work it out per guest. IS 1172 allows 180 litres per guest per day, so a room with two guests needs 360 L. An 80-room hotel at full occupancy needs 28,800 L a day for guests, plus the restaurant at 70 L per seat and any laundry or pool.

Should a commercial building have one tank or two?

Two, or one tank with two compartments. Tanks have to be drained for cleaning and repair, and a building with a single tank has no water while that happens. Connect the pair so water flows through both in normal use.

What causes water to stagnate in a commercial tank?

Too much storage for the demand. Divide the tank volume by the daily use to get the turnover in days. Oversized tanks, low occupancy, holiday closures and a second tank that is never drawn from all leave water standing. Size to real demand, arrange the inlet and outlet so water passes through the whole tank, and flush after any long closure. If stored water already smells or tastes wrong, see why water tastes or smells bad after storage.

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Sources

• Bureau of Indian Standards, IS 1172:1993, Code of Basic Requirements for Water Supply, Drainage and Sanitation. Table 1: water requirements for buildings other than residences. Clause 5.2.1: fire demand referred to IS 9668.
• World Health Organization, Minimum water quantity needed for domestic uses, technical note for emergencies. Health centres: 5 litres per outpatient and 40–60 litres per inpatient per day.
• Water Regs UK, Cold water storage cisterns (February 2024). Lid, overflow screen, insulation, support, and outlet opposite the inlet.
• Illinois EPA, Storage Tanks. Inspection at least every five years.

A rooftop tank for a multi-storey building must satisfy three separate constraints simultaneously: enough volume for peak demand and supply gaps, enough height to generate adequate pressure at the lowest fixture, and within the structural load limit of the roof slab. Getting any one of these wrong creates problems the other two cannot fix. This article walks through each constraint with the actual numbers.

The Quick Answer

Rooftop tank sizing for multi-storey buildings follows this sequence: calculate total daily demand, add buffer for supply gaps, verify pressure at the lowest floor, and confirm the slab can carry the load. The table below gives indicative volumes for common building types.

Building type	Floors	Units/occupants	Recommended tank volume
Residential apartment	G+3	16 units / ~48 people	10,000–15,000 L
Residential apartment	G+6	28 units / ~84 people	18,000–25,000 L
Residential apartment	G+10	44 units / ~132 people	30,000–45,000 L
Mixed-use commercial	G+4	Office + 8 residential units	12,000–20,000 L
Small hotel (30 rooms)	G+5	~90 guests at peak	25,000–35,000 L

Skip the math: Use the Rooftop Tank Size Calculator to get a volume figure calibrated to your building’s actual occupancy and supply conditions.

How the Calculation Works

Step 1 — Total daily demand. Multiply occupants by per-capita consumption. For residential buildings in South Asia, 135 litres per person per day is the IS 1172 standard (Bureau of Indian Standards). For UK residential, 125 litres per person per day is the Building Regulations Approved Document G baseline (with an optional stricter standard of 110 L/person/day applied by some local planning authorities). For hotels, CIBSE Guide G recommends 200–250 litres per bed per day.

Step 2 — Buffer for supply gaps. The tank must cover the longest expected gap between supply fills. If municipal supply is 6 hours per day, the tank must hold at minimum 18 hours of demand. In areas with daily outages, a 1.5× to 2× daily demand buffer is standard practice.

Step 3 — Worked example. A G+6 residential building: 28 units, average 3 occupants each = 84 people. Daily demand at 135 L/person = 11,340 L. Municipal supply is available 8 hours/day, so the tank must bridge 16 hours. Buffer factor = 11,340 × (16/24) = 7,560 L. Rounding up to the nearest standard tank size: 10,000 L tank, typically two 5,000-litre units on the roof.

Step 4 — Pressure check. Every additional metre of head (vertical height from tank base to outlet) generates approximately 0.098 bar of pressure (9.8 kPa). A ground-floor shower typically requires a minimum of 1.0 bar (roughly 10 m of head) for adequate simultaneous-fixture performance in a multi-unit building. If the tank base sits 12 m above ground level, you have 1.18 bar at the ground floor — sufficient. If the building is only 8 m tall, gravity feed may be inadequate and a booster pump is required. For a full breakdown of how tank height converts to pressure at the outlet, see our guide on hydrostatic pressure and tank design.

Key Variables That Change the Answer

Supply reliability. Buildings in cities with 24/7 mains supply can size their rooftop tank at just 50% of daily demand — it is a buffer, not primary storage. Buildings in areas with 4–8 hours of daily supply need to store 100–150% of daily demand. In areas with irregular or tanker-fed supply, 200–300% is not unusual.

Number of simultaneously active outlets. Peak hour demand — typically morning and evening — can be 3–4× average hourly demand. The tank must be able to supply this without running dry or requiring pump assistance. Building services engineers typically apply a simultaneous demand factor of 0.6–0.8 for residential buildings (CIBSE Guide G).

Roof slab load rating. Water weighs 1 kg per litre. A 10,000-litre tank weighs 10,000 kg plus the tank structure itself (200–800 kg depending on material). That is 10–11 tonnes concentrated on a small footprint. Most standard flat-slab residential construction in South Asia is designed for 150–200 kg/m². A tank footprint of 4 m × 2 m = 8 m² supporting 10,800 kg applies 1,350 kg/m² — far beyond standard slab capacity. Structural reinforcement or distribution over multiple tanks is almost always required. Use the Rooftop Load Bearing Calculator to check your own slab against a proposed tank footprint before purchasing.

Tank material and insulation. In hot climates, uninsulated black polyethylene tanks can reach internal water temperatures of 45–55°C, accelerating bacterial growth. White or insulated tanks maintain safer temperatures. HDPE is standard; FRP (fibreglass) and stainless steel are used for larger volumes or potable-grade requirements. For a comparison of how these materials perform over time, see our guide Plastic vs Steel vs Concrete Water Tanks: Full Comparison.

Building height and gravity pressure. Taller buildings have more head pressure at low floors but lower pressure at the top floors. A tank on a 10-storey building provides excellent pressure at floors 1–3 but may deliver less than 0.3 bar at floor 9. Top-floor units often require individual pressure-boosting or a separate pressure zone. Our guide on why shower pressure is low even with a full rooftop tank covers this top-floor problem specifically.

Common Mistakes in Rooftop Tank Sizing for Multi-Storey Buildings

Sizing by number of units without adjusting for actual occupancy. A 2-bedroom apartment averages 2.4 occupants in most markets — not 4 (full capacity) and not 1. Using design capacity inflates tank size; using census averages may undersize. For commercial buildings, using floor area rather than measured occupancy leads to significant errors.

Ignoring the structural load calculation. The single most common and costly mistake. Rooftop tanks are frequently installed without a structural assessment, then cracks appear in the roof slab within 2–5 years. The correct sequence is: calculate required volume, check available roof area, calculate load per square metre, compare against slab rating — before purchasing any tank. See Will My Roof Hold a Full Water Tank? for the warning signs of a slab that’s already under strain.

Placing the tank too low for gravity pressure. In buildings where the tank sits on the same floor level as the top occupied storey, gravity pressure is near zero. This is common in retrofits where structural concerns limit tank height. Without at least 2–3 m of head above the highest fixture, a booster pump is non-negotiable.

Ignoring evaporation and contamination in open tanks. Open-top rooftop tanks in hot, dusty, or high-UV environments degrade water quality rapidly. All potable rooftop tanks must be covered and ventilated with mesh screens. This is a health requirement, not an aesthetic preference.

Related Calculators You Might Need

Before finalising your tank volume, run the Rooftop Load Bearing Calculator to confirm your slab can take the load — this should happen before any purchase decision. If you need to verify the pressure reaching top-floor units, the Water Pressure Calculator converts tank height to bar or PSI. For the tank volume itself, the Rectangular Water Tank Volume Calculator and Cylindrical Tank Volume Calculator let you verify the dimensions of tanks you are comparing. And if you are evaluating the minimum height needed for shower pressure, use the Minimum Tank Height for Shower Pressure Calculator to get the exact figure for your building.

Frequently Asked Questions

How do I calculate the rooftop tank size for a 10-storey apartment building?

Multiply total occupants by per-capita daily consumption (use 135 L/person/day for South Asia per IS 1172, or 125 L/person/day for the UK per Approved Document G). Multiply the result by a supply buffer factor of 1.5–2.0 based on how many hours per day mains supply is available. Then verify the structural load of the selected tank volume against your roof slab rating — this step is frequently skipped and frequently causes structural damage.

What is the minimum tank height needed for gravity-fed water pressure?

Every metre of vertical height between the tank base and the outlet delivers approximately 0.098 bar of pressure. To meet the minimum 1.0 bar required for most showers and fixtures, the tank base must be at least 10.2 m above the fixture. For the top floor of a building, this is often physically impossible without a booster pump. Use the Minimum Tank Height for Shower Pressure Calculator for your specific installation height.

How many tanks should I install on the roof versus one large tank?

Multiple smaller tanks are usually preferable for load distribution. Two 5,000-litre tanks placed symmetrically distribute weight more efficiently across the roof slab than one 10,000-litre unit. Redundancy is also an operational benefit — if one tank fails or requires cleaning, supply continues through the other. The structural advantage is significant: distributed load vs. point load. See our guide Single Large Tank vs Multiple Small Tanks for a fuller comparison.

How much does a full rooftop tank weigh?

Water weighs exactly 1 kg per litre, regardless of tank material. A 5,000-litre HDPE tank weighs approximately 5,080–5,150 kg when full (tank shell adds 80–150 kg). A 10,000-litre FRP tank weighs approximately 10,200–10,800 kg (shell adds 200–800 kg). Add the base frame or support structure. This total load, divided by the tank footprint area, gives the kg/m² figure to compare against your slab rating — the Water Tank Weight Calculator does this calculation for your specific tank.

Can I add a second rooftop tank to an existing building?

Only after a structural assessment confirms the slab can carry the additional load. The assessment must account for the existing tank load, any other rooftop equipment, and the new addition. In many older residential buildings, the slab is already at or near capacity with the original tank. Adding more capacity underground and pumping up is often the structurally safer solution — see our guide on overhead vs underground water tanks for the trade-offs involved.

The volume of a cylindrical water tank is π × r² × h, where r is the internal radius and h is the internal height. A tank with an internal diameter of 1.2 m and height of 1.5 m holds approximately 1,696 litres. This article covers the full formula, unit conversions, worked examples in both metric and imperial, and the key measurement errors that cause tank volume to be miscalculated. If you’re deciding between a cylindrical and rectangular tank in the first place, our comparison guide Round vs Rectangular Water Tank is worth reading first.

The Quick Answer

Volume (m³) = π × r² × h  |  Volume (litres) = π × r² × h × 1,000

Where r = internal radius in metres, h = internal height in metres, and π = 3.14159.

Internal diameter (m)	Internal height (m)	Volume (litres)	Volume (gallons)
0.60 m	1.00 m	283 L	74.7 gal
0.90 m	1.00 m	636 L	168 gal
1.00 m	1.20 m	942 L	249 gal
1.20 m	1.50 m	1,696 L	448 gal
1.50 m	2.00 m	3,534 L	933 gal
2.00 m	2.50 m	7,854 L	2,075 gal
2.44 m (8 ft)	3.05 m (10 ft)	14,234 L	3,760 gal

Skip the math: Use the Cylindrical Tank Volume Calculator to enter your dimensions and get volume in litres, gallons, or cubic metres instantly.

How the Calculation Works

The formula:  V = π × r² × h

Where V is volume, r is the radius (half the internal diameter), and h is the internal height of the tank.

Worked example — metric: A vertical cylindrical tank has an internal diameter of 1.4 m and an internal height of 1.8 m.

Step 1: Find the radius. r = 1.4 ÷ 2 = 0.7 m

Step 2: Square the radius. r² = 0.7 × 0.7 = 0.49 m²

Step 3: Multiply by π. π × 0.49 = 1.5394 m²

Step 4: Multiply by height. 1.5394 × 1.8 = 2.771 m³

Step 5: Convert to litres. 2.771 × 1,000 = 2,771 litres

Worked example — imperial: A tank is 4 feet in diameter and 5 feet tall.

Step 1: Radius = 4 ÷ 2 = 2 feet

Step 2: r² = 4 ft²

Step 3: π × 4 = 12.566 ft²

Step 4: 12.566 × 5 = 62.83 cubic feet

Step 5: Convert to US gallons. 62.83 × 7.481 = 470 US gallons (approximately 1,779 litres)

Key Variables That Change the Answer

Internal vs external dimensions. Tank manufacturers typically list external dimensions on product labels. Wall thickness on HDPE tanks is commonly 6–12 mm; on FRP tanks, 8–16 mm; on steel tanks, 3–6 mm. For a tank with a 1.2 m external diameter and 10 mm wall thickness, the internal radius is 0.59 m, not 0.60 m. Over the height of the tank, this reduces volume by approximately 3.3% — significant when comparing to a stated 1,000-litre capacity.

Tank orientation. Vertical cylinders use the formula above. Horizontal cylinders use a different formula because the liquid level changes the cross-sectional area of the wetted region — a horizontal tank at 50% fill is not at 50% volume. For horizontal tanks, use the Horizontal Tank Volume Calculator which applies the correct partial-fill formula.

Domed or conical ends. Many cylindrical tanks have a domed or conical top or base. These add volume beyond the pure cylinder calculation. A hemispherical dome on a 1.2 m diameter tank adds approximately 452 litres of additional volume. Tanks with conical bottoms have less usable volume than their total volume suggests — the cone below the outlet level is dead space.

Measurement method. Measuring with a tape measure over the outside of an installed tank introduces error from curvature, insulation, and operator technique. For accurate results, measure height with a rigid ruler dropped vertically through the inspection hatch, and diameter at the widest internal point. Do not assume external measurements are internal measurements.

Common Mistakes When Calculating Cylindrical Tank Volume

Using diameter instead of radius in the formula. This is the single most common error. V = π × d² × h (where d is diameter) overstates volume by a factor of 4. The correct formula always uses radius (half the diameter). A tank calculated as 4,000 litres this way actually holds 1,000 litres.

Measuring in mixed units. Entering diameter in centimetres and height in metres without converting both to the same unit produces a wildly incorrect answer. Always convert all measurements to the same unit before calculating. If working in centimetres, the result is in cm³ — divide by 1,000,000 to get litres (1 litre = 1,000 cm³, so cm³ ÷ 1,000 gives millilitres, and ÷ 1,000 again gives litres).

Ignoring the dead zone. A cylindrical tank’s usable volume is not its geometric volume. If the outlet fitting sits 80 mm above the floor, and the tank has a 1 m radius, the dead volume below the outlet is π × 1² × 0.08 = 251 litres. For a 2,000-litre tank, that is 12.5% of rated capacity that can never be used.

Assuming nominal rated volume matches geometry. Tank manufacturers’ stated volumes are nominal — often the rounded figure from the geometric calculation, sometimes slightly understated as a conservative rating, occasionally based on external dimensions by mistake. Always verify using the formula before making supply decisions.

Related Calculators You Might Need

For partially filled cylindrical tanks — where you need to know how much water is currently in the tank based on a measured depth — the Cylindrical Tank Volume Calculator supports partial-fill calculations. If you need to convert your result from litres to gallons, use the Tank Litres to US Gallons Converter or the Tank Litres to UK Gallons Converter, depending on which gallon standard you need. For tanks with more complex geometries, the Capsule Tank Volume Calculator handles cylinders with hemispherical ends, and the Cone Bottom Tank Volume Calculator handles tanks with conical bases. If your tank is rectangular instead, see our companion guide on calculating the volume of a rectangular water tank. Once you know the volume, the Water Tank Weight Calculator converts this to the structural load your installation must support.

Frequently Asked Questions

What is the formula for the volume of a cylindrical water tank?

V = π × r² × h, where r is the internal radius (half the internal diameter) and h is the internal height. The result is in cubic units — multiply by 1,000 to convert from m³ to litres, or by 7.481 to convert ft³ to US gallons. Use the Cylindrical Tank Volume Calculator to avoid manual calculation errors.

How do I calculate the volume of a horizontal cylindrical tank?

For a full horizontal cylinder, use the same formula: V = π × r² × L (where L is the length). However, if the tank is partially filled, the formula changes because the wetted cross-section is no longer a full circle. You need to calculate the area of a circular segment, which requires the height of the liquid level. This is why a separate calculator is used for horizontal tanks at partial fill — the formula is significantly more complex.

How many litres does a cylinder 1m diameter and 2m high hold?

V = π × 0.5² × 2 = π × 0.25 × 2 = 1.5708 m³ = 1,571 litres. Note that this is the geometric volume — subtract any dead zone below the outlet to get usable volume.

How do I convert cubic metres to litres for a water tank?

Multiply by exactly 1,000. One cubic metre = 1,000 litres. This is exact, not an approximation — 1 litre is defined as 0.001 m³ by the International System of Units (SI). Similarly, 1 cubic foot = 28.317 litres, and 1 US gallon = 3.785 litres.

Does the shape of the tank bottom affect the volume calculation?

Yes. A flat-bottom cylinder uses V = π × r² × h exactly. A domed base adds volume (a hemispherical dome of radius r adds (2/3)πr³). A conical base reduces usable volume — if the cone apex is at the outlet, all of the cone volume is dead space. For tanks with non-flat ends, the total geometric volume exceeds the simple cylinder formula, but usable volume depends on outlet placement.

A 500-litre tank used by a household of four consuming 50 litres per person per day will last exactly 2.5 days. That is the entire calculation — tank volume divided by daily consumption. This article breaks down how to get both numbers right, what variables compress or extend your supply, and the mistakes that cause people to run dry earlier than expected.

The Quick Answer

Tank duration (days) = Tank volume (litres) ÷ Daily consumption (litres/day). For most planning purposes, WHO guidance sets minimum survival water needs at 15 litres per person per day; FEMA recommends at least 3.78 litres (1 US gallon) per person per day for short-term emergency storage, though this covers only drinking and basic sanitation.

Household size	Daily use (litres)	500 L lasts	1,000 L lasts	2,000 L lasts
1 person	50 L	10 days	20 days	40 days
2 people	100 L	5 days	10 days	20 days
4 people	200 L	2.5 days	5 days	10 days
6 people	300 L	1.7 days	3.3 days	6.7 days
Emergency (15 L/person × 4)	60 L	8.3 days	16.7 days	33.3 days

Skip the math: Use the How Long Will My Tank Last Calculator to enter your exact tank size and household consumption for an instant result.

How the Calculation Works

Formula:  Days of supply = Usable tank volume (L) ÷ Total daily demand (L/day)

Worked example: A family of 5 in a warm climate uses an average of 65 litres per person per day — cooking, bathing, toilet flushing, and laundry. Their tank is a 2,000-litre overhead tank, but the outlet is 50 litres above the tank floor, so usable volume is 1,950 litres.

1,950 ÷ (5 × 65) = 1,950 ÷ 325 = 6 days of supply.

If their municipal supply fails, they have six days before the tank runs dry — assuming no wastage. Add a 20% safety margin (standard practice in building services engineering) and the effective planning duration becomes 4.8 days before they need to act.

Key Variables That Change the Answer

Usable vs total volume. Most tanks have a dead zone at the bottom — sediment, the physical position of the outlet pipe, or a valve installed 50–100 mm above the floor. Subtract this from your nominal capacity. On a 1,000-litre tank, the dead zone can be 30–80 litres depending on design.

Per-capita consumption varies significantly by context. In Australia, average household consumption runs 200–250 litres per person per day (Water Services Association of Australia, 2023). In sub-Saharan Africa, per-capita access can be under 20 litres/day. If you are sizing for backup rather than primary supply, use your actual metered daily consumption, not a regional average.

Seasonal demand shifts. In summer, outdoor watering, cooling, and higher personal hygiene use can push consumption up 30–50% above winter baseline. A tank that lasts 10 days in January may last only 6–7 days in July in a hot climate.

Top-up frequency. If your municipal supply is intermittent — common across South Asia, parts of Africa, and the Middle East — your effective tank duration is not full drain to empty. It is the gap between supply windows. Design your tank size around the longest expected gap, not the average.

Evaporation and leakage. Uncovered tanks in hot climates can lose 1–3% of volume per week to evaporation. A slow float valve leak can waste 20–50 litres per day without being audible. These losses silently reduce your effective duration.

Common Mistakes When Estimating Tank Duration

Using nominal tank volume without subtracting dead space. A tank labelled 1,000 litres delivers 1,000 litres only if the outlet is at the absolute base. In practice, 5–8% is often inaccessible. This matters most with smaller tanks where the error is proportionally larger. If your tank seems to be emptying faster than these numbers suggest it should, our troubleshooting guide on why your water runs out in 2 days covers the likely causes.

Using per-capita averages from the wrong context. Applying a 250-litre Australian average to a household that actually uses 90 litres per person will cause significant overbuilding — and vice versa. Pull your actual water meter reading over two to four weeks, divide by occupants and days. That number is your real input.

Ignoring children and elderly occupants. Caregiving for infants (formula preparation, frequent washing) and elderly individuals can add 20–40 litres per day per dependent beyond base adult consumption. Household composition matters.

Calculating for normal use during an emergency. During supply failures, toilet flushing (which accounts for 25–30% of typical home water use per UK Environment Agency data) often gets reduced or eliminated. Factoring in emergency-mode behaviour can stretch a 5-day tank to 7–8 days without any additional capacity. For US households planning around this specifically, see our guide on emergency water storage and FEMA guidelines.

Related Calculators You Might Need

Once you know how long your current tank lasts, the logical next step is confirming whether that duration is sufficient — or sizing a new tank to meet a specific backup target. The Water Tank Size for Home Calculator lets you work in reverse: enter your household size and required backup days to get a recommended tank volume, and our guide on how to tell if your water tank is the wrong size for your home covers the warning signs. If you are sizing for an emergency scenario specifically, the Emergency Water Storage Calculator applies FEMA and WHO-based minimums to your household. For apartment buildings with multiple units, the Apartment Water Tank Size Calculator handles multi-unit demand stacking. And before committing to a tank volume, check your Daily Water Requirement Calculator to verify your consumption figure is accurate.

Frequently Asked Questions

How long does a 1000-litre water tank last for a family of 4?

At typical South Asian or African consumption of 50 litres per person per day, a 1,000-litre tank lasts 5 days for a family of four. At Australian or UK average consumption of 150–200 litres per person per day, the same tank lasts 1.25–1.67 days. The number is entirely dependent on your actual daily demand — use your water meter to measure it rather than relying on regional averages.

What is the formula to calculate water tank duration?

Duration (days) = Usable tank volume (litres) ÷ Daily household consumption (litres/day). Usable volume is your tank’s total capacity minus the dead zone at the bottom (typically 3–8% of total). Use the How Long Will My Tank Last Calculator to run this automatically with your specific inputs.

Does tank shape affect how long it lasts?

Shape does not change duration directly — only usable volume matters. However, tanks with conical or sloped bottoms have smaller dead zones than flat-bottomed rectangular tanks, so they yield slightly more usable volume per rated capacity. A cone-bottom tank rated at 1,000 litres may give you 960–980 litres of usable water versus 920–950 litres from a flat-bottom design.

How do I account for leaks in my tank duration calculation?

Add a 10–15% wastage buffer to your daily consumption figure. If you suspect a leak, measure the tank level at the same time on two consecutive days when no one is using water (e.g., overnight). Any unexplained drop in litres divided by the number of hours between readings gives you the leak rate. A 20-litre drop measured over an 8-hour overnight period is 2.5 litres per hour, or roughly 60 litres per day of loss — a problem that eliminates the usefulness of any size calculation.

How long can water be stored in a tank before it goes bad?

Treated municipal water stored in a clean, covered tank typically remains safe for 6–12 months under cool, dark conditions. Without treatment, storage life drops to 2–3 days in warm climates. UV exposure, algae, and bacterial growth are the primary risks. The WHO recommends maintaining a free chlorine residual of 0.2–0.5 mg/L in stored water for ongoing safety.

A family of four needs a water tank sized between 2,000 and 5,000 litres (530–1,320 US gallons), depending on how reliable your supply is, how many days of backup you want, and your household’s actual consumption. The WHO baseline is 50 litres per person per day for basic needs; real household use in developed countries runs 100–200 litres per person per day. This article walks through the calculation, the variables that shift it, and the most common sizing mistakes so you can commit to a number with confidence.

The quick answer

At 50 L/person/day (WHO minimum), four people need 200 L/day. At 150 L/person/day (typical urban household), they need 600 L/day. The tank size you need depends on how many days of backup storage you require — a single-day buffer is useless if your supply cuts out for three days. If you’re specifically working out a 3-day backup figure, see our guide on how much water storage you need for 3 days of backup.

Usage level	L/person/day	1-day buffer	3-day buffer
Minimum (WHO)	50	200 L	600 L
Moderate	100	400 L	1,200 L
Typical urban	150	600 L	1,800 L
High-use household	200	800 L	2,400 L

Skip the math: Use the water tank size for home calculator to get a figure tailored to your household’s actual consumption and supply conditions.

How the calculation works

The formula is straightforward:

Tank size (L) = persons × daily use (L/person/day) × backup days + safety buffer (10–15%)

For a family of four at 150 L/person/day wanting three days of storage:

4 × 150 × 3 = 1,800 L. Add a 10% safety buffer: 1,800 × 1.10 = 1,980 L. Round up to a standard 2,000 L tank.

That 10% buffer matters. Tanks are measured at full capacity but operated below it — sediment accumulates at the bottom, and most tank gauges lose accuracy in the bottom 5–10% of the tank. You’ll also never get a delivery that tops it up to exactly zero headroom.

Key variables that change the answer

Supply reliability. If your mains supply fails daily for 4–6 hours (common in parts of South Asia and Sub-Saharan Africa), you need at minimum 1 day’s buffer. In areas with weekly or irregular supply, plan for 5–7 days of storage — that moves a family-of-four tank into the 4,000–7,000 L range.

Climate and seasonal demand. Household water use rises 20–40% in hot months due to increased bathing, garden watering, and evaporative cooling. In hot climates, size the tank against peak-season demand, not the annual average.

Household composition. A family of four with two teenagers uses significantly more than one with two young children. Teenagers average 20–30% higher water use than children under 10 (IWA household benchmarks).

Appliance efficiency. Low-flow showerheads cut shower consumption from ~15 L/min to ~7 L/min. A household with water-efficient appliances throughout can drop daily per-person use from 150 L to 90–100 L — a meaningful difference when sizing for 5+ days of backup.

Tank placement. Rooftop tanks are limited by structural load capacity. A 2,000 L tank full of water weighs roughly 2,100 kg including the tank shell. If your roof wasn’t engineered for that load, you’re constrained regardless of what the calculation says — see will my roof hold a full water tank before committing to a size.

Family of 4 sizing scenarios

Scenario	Daily use	Backup days	Recommended tank size
Urban, reliable supply	150 L/person	1 day	660 L (round to 1,000 L)
Urban, intermittent supply	150 L/person	3 days	2,000 L
Rural, borehole or irregular tanker	100 L/person	7 days	3,000–3,500 L
Off-grid or emergency prep	50–80 L/person	14 days	3,000–4,900 L

Common mistakes

Sizing for minimum rather than typical use. Many households size for the WHO’s 50 L/person/day because it looks conservative. That’s a basic-needs figure — drinking, cooking, basic sanitation. It excludes laundry, dishwashing, showers, and garden use. A family of four sized to 50 L/person will drain a 1,000 L tank in five days during normal operation, which is 2–3 days less than they expected.

Ignoring the no-supply scenario duration. Buyers in areas with intermittent supply ask ‘how much for a family of four?’ without specifying backup days. A 1,000 L tank and a 5,000 L tank are both ‘for a family of four’ — the difference is whether you’re sizing for 1-day inconvenience or week-long outages. Know your worst-case supply gap before committing to a size. If you’re unsure whether your current tank is already the wrong size, our guide on how to tell if your water tank is the wrong size for your home walks through the warning signs.

Forgetting structural limits before buying. A 3,000 L rooftop tank weighs over 3 tonnes when full. Residential rooftop slabs in many regions are designed for 150–200 kg/m². Placing an oversized tank on an inadequately reinforced roof causes structural damage over time — cracking, deflection, and in extreme cases, collapse. Check the load rating first.

Not accounting for dead volume. The bottom 5–10% of a tank is typically unusable due to sediment accumulation. A nominally 2,000 L tank delivers closer to 1,800 L of usable water. Size up by at least 10% to account for this, which the safety buffer in the formula above captures — but only if you actually apply it.

Related calculators you might need

Once you have a tank size in mind, the next question is usually whether your roof can support it. Use the rooftop load bearing calculator to check your slab’s capacity before purchasing. If you’re on intermittent supply, the how long will my tank last calculator lets you enter your tank size and daily consumption to find out exactly how many days it covers. For daily use estimates you haven’t nailed down yet, the daily water requirement calculator walks through each household activity and totals it up. And if you’re also considering water treatment after storage, the chlorine dosage calculator gives you the correct disinfection amount based on your tank’s volume.

Frequently asked questions

What size water tank do I need for a family of 4?

For a typical family of four using 150 L/person/day with three days of backup, you need approximately 2,000 litres (530 US gallons). If your supply is reliable and you only need one day’s buffer, 600–1,000 litres is sufficient. In areas with weekly supply interruptions, size up to 3,500–4,000 litres. Use the water tank size for home calculator to model your specific scenario.

How many litres of water does a family of 4 use per day?

In developed countries, the average is 100–200 litres per person per day, putting a family of four at 400–800 litres daily. In regions with limited infrastructure, usage drops to 50–80 litres per person. The WHO emergency minimum is 15 litres per person per day for survival. Actual household use varies significantly with appliance efficiency and lifestyle — teenagers and frequent bathers push that number toward the higher end.

Is a 1,000-litre tank enough for a family of 4?

Only if your mains supply is reliable and interruptions are rare. At 150 L/person/day, a family of four drains 600 litres daily — a 1,000 L tank gives you fewer than two days of backup. For areas with regular outages, 1,000 litres is undersized. If supply is consistent and the tank is just a pressure buffer, 1,000 litres can work.

How do I calculate my family’s daily water use?

Add up each activity: toilet flushing (6–9 L per flush, average 5 flushes/person/day), showers (8–15 L/min for 5–10 minutes), laundry (50–80 L per load), dishwashing (15–35 L per cycle), drinking and cooking (5–10 L/person/day). Total it per person, then multiply by four. Most households land between 100 and 180 litres per person per day through this method. For a fuller breakdown, see the how to size a water tank for daily household use guide.

Does tank size affect water pressure?

For gravity-fed systems, yes. Pressure is determined by the height of the water above the outlet, not the tank’s volume. A 2,000 L rooftop tank at 5 metres height generates approximately 0.5 bar of pressure — adequate for showers but marginal for high-pressure appliances. A larger tank at the same height gives no additional pressure. If pressure is a concern, focus on elevation rather than volume.

How often does a 2,000-litre tank need to be refilled at normal use?

A family of four at 150 L/person/day consumes 600 litres daily, meaning a 2,000 L tank lasts roughly three days at full capacity. Accounting for dead volume (10%), usable capacity is closer to 1,800 L — just under three days. If supply refills the tank partially each day, that gap extends indefinitely; if supply is batch-delivered, plan refills every 2–3 days.

Sizing a household water tank correctly comes down to two numbers: daily consumption and backup days required. Multiply them, add a 10% buffer, and you have your minimum tank size. The difficulty is pinning down accurate consumption figures — most households underestimate by 30–50% compared to actual metered use. This article gives you the formula, the per-activity benchmarks to estimate consumption accurately, and the supply-scenario adjustments that change the answer significantly.

The quick answer

Use this formula:

Tank size (L) = persons × L/person/day × backup days × 1.10

The 1.10 factor adds a 10% safety buffer to account for dead volume, sediment, and gauge inaccuracy. If you want a fuller discussion of why “3 days” is a common backup target, see our guide on how much water storage you need for 3 days of backup.

Household size	L/person/day	1-day buffer	3-day buffer
1 person	100–150	110–165 L	330–495 L
2 persons	100–150	220–330 L	660–990 L
4 persons	100–150	440–660 L	1,320–1,980 L
6 persons	100–150	660–990 L	1,980–2,970 L

Skip the math: use the water tank size for home calculator to enter your household details and get a tailored recommendation immediately. If you have exactly 4 people in your household, our guide on what size water tank a family of 4 needs works through this specific case.

How the calculation works

Daily consumption per person is the key variable. The benchmark figures used in the table above come from IWA (International Water Association) household use studies across urban residential properties. Here is where that 100–150 L/person/day goes:

Activity	Litres per use	Daily total (per person)
Toilet flushing	6–9 L per flush	30–45 L
Shower (5 min, standard head)	60–75 L	60–75 L
Shower (5 min, low-flow head)	35–40 L	35–40 L
Drinking & cooking	—	5–10 L
Laundry (shared, 1 load/day)	50–80 L per load	12–20 L
Dishwashing (machine)	15–20 L per cycle	4–7 L
Hand washing, misc.	—	5–10 L

A household with standard-efficiency appliances and typical shower habits lands at 120–150 L/person/day. Fitting low-flow showerheads and a water-efficient washing machine drops this to 90–110 L/person/day — a reduction that allows sizing down by 20–25% on the same backup target.

Worked example: 4 people, 3-day backup, standard appliances

Daily use: 4 × 140 L = 560 L/day. Target: 3 days. Raw figure: 560 × 3 = 1,680 L. Add 10% buffer: 1,680 × 1.10 = 1,848 L. Round up to the nearest standard size: 2,000 L tank. For help matching a raw figure like this to what’s actually sold, see our comparison guide 1000L vs 2000L vs 5000L Water Tank: Which Size Is Right?

Key variables that change the answer

Supply reliability. This is the single biggest factor. In cities with reliable 24-hour mains supply, a tank sized for 1 day’s buffer is a pressure-head buffer, not emergency storage — 500–800 L is adequate for most households. In areas where supply runs for 4–6 hours per day, you need enough to cover the off-supply window plus a margin — typically 2–3 days. Where tanker deliveries are the only source and arrive weekly or fortnightly, size for the full delivery interval: 7–14 days of storage.

Rooftop vs underground placement. Rooftop tanks are governed by structural load limits. A full 2,000 L tank weighs roughly 2,100 kg — most residential slabs rated for rooftop loads in developing markets are engineered for 200–300 kg/m². On a 2 m × 1.5 m footprint (3 m²), that’s 600–900 kg maximum — a tank in the region of 600–800 L without structural assessment, once you allow for the tank’s own weight. Underground tanks have no such constraint and are the right choice for larger household storage volumes. Our guide Overhead vs Underground Water Tank: Which Is Better? compares both options in full, and the Rooftop Load Bearing Calculator lets you check your own slab’s capacity before buying.

Seasonal demand variation. Hot climates see water use spike 25–40% in summer — more frequent showers, garden irrigation, evaporative coolers. If you’re sizing for a region with hot dry seasons, apply your peak-season consumption figure, not the annual average. A tank sized for average demand runs dry during the months you need it most.

Household type. Young children (under 10) use 30–40% less water than adults. Elderly residents use less shower water but more frequent toilet flushes. Teenagers and young adults are consistently the highest per-person users. Weighting your household composition against these patterns gives a more accurate daily total than simply multiplying people by a fixed per-person average.

Common mistakes

Using the 15 L/person/day emergency survival figure for tank sizing. The commonly cited 15 L/person/day emergency threshold (used in humanitarian relief standards) covers drinking and basic sanitation only. It does not include showers, laundry, or dishwashing. Using it to size a residential household tank produces a tank that runs out in days under normal operation. The correct baseline for residential sizing is 100 L minimum; 140–160 L for a typical household.

Ignoring the supply pattern and sizing by household size alone. A 1,000 L tank serves a family of two with reliable mains and a family of two in a water-scarce region very differently. Without factoring in supply frequency, tank size recommendations are meaningless. Define your worst-case supply gap first, then size.

Underestimating garden and outdoor use. Garden irrigation, car washing, and outdoor cleaning add 50–150 L/day to a household’s consumption during active months. This frequently pushes per-person consumption above 200 L/day for households with moderate-sized gardens. Exclude it only if your outdoor area is fully paved or you source outdoor water separately.

Not accounting for tank accessibility for maintenance. Tanks that are never fully drained and cleaned accumulate biofilm and sediment — reducing usable capacity and degrading water quality over 2–5 years. An undersized tank that’s topped up continuously without ever being emptied causes more problems than one correctly sized and periodically cleaned. Factor in a cleaning interval (every 6–12 months) when planning tank size — see our guide on why there’s sediment at the bottom of your water tank for what to look for.

Related calculators you might need

If you don’t yet know your daily consumption, the daily water requirement calculator walks through each household activity to build a total. Once you’ve settled on a tank size, the how long will my tank last calculator confirms how many days it covers at your consumption rate. For underground or larger tanks, check the total filled weight with the water tank weight calculator before finalising placement. And if your area has treated water supply, the safe water storage duration calculator tells you how long stored water remains safe before requiring retreatment.

Frequently asked questions

How do I size a water tank for daily household use?

Multiply the number of people in your household by litres per person per day (use 140–160 L for a standard household), then multiply by your backup days, and add 10%. For a family of four needing 3 days’ backup: 4 × 150 × 3 × 1.10 = 1,980 L — round to a 2,000 L tank. Use the water tank size for home calculator to refine this for your specific usage pattern.

What is the minimum water tank size for a household?

The minimum that makes practical sense is 500–1,000 L for a single person or couple with reliable mains supply — enough to buffer daily supply interruptions. Below 500 L, a tank provides almost no meaningful backup. For a family of four with any supply variability, 2,000 L should be considered the floor, not the target.

How much water does a household use per day?

In OECD countries, average residential water use is 100–200 litres per person per day. In urban South Asia and Sub-Saharan Africa, actual delivered supply is often 40–80 L/person/day due to infrastructure limits. Your actual number depends on appliance efficiency, climate, and lifestyle — the per-activity breakdown in the table above will give you a household-specific total.

How many days should a household tank last?

Size for your realistic worst-case supply gap, not the average. If your area loses supply for one day twice a week, size for 2–3 days. If tanker deliveries are the only source and arrive every 7–10 days, size for the full interval. A tank that runs dry before the next refill provides no safety margin — add at least 1.5–2 days beyond your expected gap.

Does a larger tank mean better water pressure?

Not from volume — pressure in a gravity-fed system depends on the height of the water surface above the outlet, not the tank’s capacity. A 500 L tank at 5 m elevation produces the same pressure as a 5,000 L tank at 5 m elevation. To improve gravity-feed pressure, raise the tank or install a pump booster. Volume and pressure are independent variables — see our guide on hydrostatic pressure and tank design for the full explanation.

The volume of a rectangular water tank equals length × width × height, measured in the same unit throughout. The result is in cubic units — multiply by 1,000 to convert cubic metres to litres, or multiply cubic feet by 7.48 to get US gallons. This article covers the formula, unit conversions, partial-fill calculations, and the adjustments needed for tanks with wall thickness or irregular proportions.

The quick answer

For a rectangular tank, the calculation has one step:

Volume = Length × Width × Height

All three dimensions must be in the same unit before you multiply. Use metres for litres, feet for cubic feet, or inches for cubic inches (then convert). Here are the most common unit conversions:

Measurement unit	Volume result	Convert to litres by
Metres (m)	Cubic metres (m³)	× 1,000
Centimetres (cm)	Cubic centimetres (cm³)	÷ 1,000
Feet (ft)	Cubic feet (ft³)	× 28.317
Inches (in)	Cubic inches (in³)	÷ 61.024

Skip the conversion math: the rectangular water tank volume calculator handles all unit conversions and outputs results in litres, gallons, and cubic metres simultaneously.

How the calculation works

The formula treats the interior of the tank as a cuboid. Each dimension — length (L), width (W), height (H) — contributes directly and proportionally to the result. Double the height, you double the volume.

Worked example: residential rooftop tank

A concrete rooftop tank is 2.5 m long, 1.5 m wide, and 1.2 m high.

Volume = 2.5 × 1.5 × 1.2 = 4.5 m³ = 4,500 litres

At 150 litres per person per day, this serves a household of four for 7.5 days without refill.

Worked example: US gallon conversion

A tank measuring 8 ft × 4 ft × 3 ft:

Volume = 8 × 4 × 3 = 96 ft³ × 7.48 = 718 US gallons (2,718 litres)

Key variables that change the answer

Wall thickness. Many concrete and brick tanks have walls 100–200 mm thick. If you measure the external dimensions, you need to subtract wall thickness from each side to get the internal dimensions. For a tank with 150 mm walls: a 2.5 m external length becomes 2.5 − 0.30 = 2.2 m internally. Ignoring this on a brick tank can overstate volume by well over 20% on smaller tanks, less on larger ones — always calculate from internal dimensions.

Fill level. Tanks are almost never filled to capacity. If you’re calculating usable volume or checking how much water is currently stored, use the actual fill height rather than the tank height. Volume at partial fill = L × W × fill depth.

Freeboard. Rooftop and open-top tanks are typically filled to 50–100 mm below the rim to prevent overflow and splashing. On a standard 1.2 m tank, that’s a 4–8% reduction in effective height. On the 2.5 × 1.5 m tank in the example above, that works out to roughly 190–375 litres of usable capacity lost — not trivial if you’re sizing tightly.

Internal fittings. Tanks with inlet baffles, sediment chambers, or structural pillars have obstructions that displace water. Commercial and agricultural tanks sometimes have internal dividers — measure the usable chambers separately and sum them.

Rectangular vs other tank shapes

Tank shape	Formula	Best use	Volume efficiency
Rectangular	L × W × H	Rooftop, underground, concrete	100% space used
Cylindrical	π × r² × H	Polyethylene tanks, silos	~79% of bounding box
Horizontal cylinder	π × r² × L	Underground, transport	~79% of bounding box
Cone-bottom	Cylinder + 1/3 cone	Agriculture, mixing	Varies

Rectangular tanks are the most space-efficient shape — they fill every cubic centimetre of the space they occupy. A cylindrical tank of the same external dimensions holds only 78.5% as much water.

Common mistakes

Measuring external instead of internal dimensions. For plastic tanks this barely matters — walls are thin. For concrete, brick, or fibreglass tanks with walls 100–200 mm thick, external measurement can meaningfully overstate volume, especially on smaller tanks. Always measure from inside face to inside face.

Mixing units mid-calculation. Multiplying 2 metres × 150 centimetres × 1,200 millimetres is a common error — the dimensions aren’t in the same unit. Convert everything to one unit first. The most reliable approach: use metres throughout for litres, or centimetres throughout and divide the cm³ result by 1,000.

Using nominal tank capacity instead of calculating it. Tank manufacturers list nominal capacities that sometimes reflect design intent rather than actual internal volume. A ‘5,000 L’ tank measured physically may hold 4,750 L due to wall thickness, baffles, or manufacturing tolerances. For critical sizing — hospital storage, agricultural irrigation — always calculate from measured dimensions rather than the label.

Ignoring the dead zone at the bottom. Sediment accumulates at the base of rectangular tanks over time, particularly in concrete tanks supplied by surface water. The bottom 50–100 mm is effectively dead volume. A 2.5 × 1.5 m tank loses 188–375 litres to sediment accumulation — deduct this from usable capacity in your planning.

Related calculators you might need

If your tank isn’t a simple rectangle — it has a sloped bottom, a cone section, or it’s cylindrical — the cylindrical tank volume calculator or the cone bottom tank volume calculator will handle the correct geometry. If you’ve calculated litres and need US or imperial (UK) gallons for purchasing or regulatory purposes, use the litres to US gallons converter or the litres to UK gallons converter. And if you’re installing a rectangular concrete tank on a rooftop, cross-check the filled weight against your slab rating using the water tank weight calculator before pouring or placing.

Frequently asked questions

How do I calculate the volume of a rectangular water tank?

Multiply length × width × height, with all three dimensions in the same unit. If using metres, the result is cubic metres — multiply by 1,000 for litres. Example: a tank 2 m × 1 m × 1.5 m holds 3 m³ or 3,000 litres. Use the rectangular water tank volume calculator for instant results across multiple units simultaneously.

What is the formula for tank volume in litres?

Volume (litres) = Length (m) × Width (m) × Height (m) × 1,000. Alternatively, measure in centimetres: Volume (litres) = L (cm) × W (cm) × H (cm) ÷ 1,000. Both produce the same result. For feet and gallons: L (ft) × W (ft) × H (ft) × 7.48 = US gallons.

How do I calculate how much water is in my rectangular tank?

Measure the current depth of water in the tank (the fill level, not the tank height). Then: Volume = Length × Width × Fill depth. All measurements in the same unit. If your 2.5 × 1.5 m tank currently has water 0.8 m deep: 2.5 × 1.5 × 0.8 = 3 m³ = 3,000 litres currently stored.

Does wall thickness affect the calculation?

Yes, for any tank with walls thicker than 20 mm — concrete, brick, fibreglass, or thick HDPE. Subtract twice the wall thickness from each external dimension to get internal dimensions. A tank with 150 mm walls has 300 mm (0.3 m) deducted from both length and width. Ignoring this on a large concrete tank can overstate volume by hundreds of litres.

How accurate is the rectangular tank volume formula?

Mathematically exact for a perfect cuboid with uniform walls. Real-world accuracy depends on how carefully you measure. A 10 mm error in a 1,000 mm dimension is 1% — acceptable. For a 2,500 mm length, a 25 mm measurement error creates a 1% volume error. Measure each dimension at multiple points and use the smallest reading if walls are uneven.

To size a water tank for your home, multiply the number of people by the litres each person uses per day, then by the number of days the tank must last without a refill, and add 20% as a buffer. A family of four using 135 litres each and covering a 2-day gap needs 4 × 135 × 2 × 1.2 = 1,296 litres, so the tank to buy is 1,500 litres (about 400 US gallons).

The formula is the easy part. The answer swings most on two inputs that people tend to guess: how much water the household really uses, and how long the supply really stays off. This guide shows how to pin down both, then how to check that the tank you picked will work on your roof, with your pump, inside your supply window.

The water tank size formula

Tank size (litres) = People × Litres per person per day × Backup days × 1.2

Each input has one job, and each has a right place to get it from:

Input	What it means	Where to get it
People	Everyone who lives in the home, plus live-in staff and regular guests	Count the household you expect in a few years, not only today
Litres per person per day	Average daily use for drinking, cooking, bathing, toilets and laundry	Your water meter or bill (Step 1)
Backup days	How long the tank must supply the house with nothing coming in	Your longest normal gap in supply (Step 2)
× 1.2	A 20% buffer on top of the base volume	Covers busy days and the water a tank cannot deliver (Step 3)

Working in gallons? Divide litres by 3.785 for US gallons or by 4.546 for UK gallons, or use the tank litres to gallons converter.

Step 1: Work out how much water your household uses

This is the input that makes or breaks the result. A guess of 80 litres per person when the real figure is 160 gives you a tank half the size you need.

The accurate way: take it from your meter

Take one month of consumption from your bill or from two meter readings, divide by the number of days, then divide by the number of people. If the bill is in cubic metres (or kilolitres), multiply by 1,000 to get litres.

Example: the meter shows 16.2 m³ used in 30 days by a household of four. 16,200 L ÷ 30 days ÷ 4 people = 135 litres per person per day.

Use a hot-season month if you can, because that is when the tank is under the most strain. If the dials confuse you, our guide on how to read a water meter walks through each type, and the Water Meter Reading Calculator turns two readings into daily use.

No meter? Start from a published benchmark

Per-person figures vary a great deal between sources because they measure different things. These are the ones worth knowing, and what each is meant for:

Litres per person per day	What the figure covers	Source
3.8 L (1 US gallon)	Emergency drinking and basic sanitation only	Ready.gov (FEMA)
15 L	Humanitarian minimum for drinking, cooking and hygiene in a crisis	Sphere Handbook
50 L	“Intermediate access”: basic household needs met, low health concern	WHO (2020)
100 L+	“Optimal access”: water piped to several taps in the home	WHO (2020)
135 L (150–200 L in large cities)	Indian design norm for homes with flush toilets; about 45 L of it is flushing	IS 1172:1993
About 140 L	Measured household average, England, 2023/24	Defra
About 310 L (82 US gallons)	Measured US home average, including garden watering	US EPA WaterSense

For a piped home with flush toilets, showers and a washing machine, 130–150 litres per person per day is a sound planning figure when you have nothing better. The Daily Water Requirement Calculator adjusts it for children, climate and how much comfort you want.

Do not size a house tank on 50 litres or on 1 gallon a day. The WHO 50-litre level describes the point where health risk becomes low, not what a piped household draws. The FEMA gallon is drinking water for a disaster. A tank sized on either runs dry in normal use. For disaster stock specifically, see our FEMA emergency water storage guide.

What pushes your number up or down

Three fixtures account for most indoor use, and the gap between efficient and older models is large:

Fixture	Efficient model	Standard or older model
Shower	7.6 L/min, about 60 L for 8 minutes	9.5 L/min, about 76 L for 8 minutes
Toilet	4.8 L per flush	6 L per flush; old cisterns can use up to about 23 L
Washing machine	About 53 L per load	About 76 L per load

Heat raises drinking and bathing, and Ready.gov notes that drinking needs alone can double in very hot weather. Garden watering is often the largest single use in a dry month, so if the garden runs off the same tank, add it on top instead of assuming the benchmark covers it.

Step 2: Decide how many backup days you need

Backup days are set by your supply, not by your household. Two identical families need very different tanks if one has water around the clock and the other gets two hours every third day. The values below are planning guides, not code requirements:

Your supply	Backup days to plan for	Why
Continuous mains, rare outages	1 day	Covers a burst main or a day of repair work
Mains every day, a few hours only	1–2 days	One missed window should not empty the tank
Mains on alternate days	2–3 days	The normal gap, plus one skipped turn
Every third day or irregular	Longest usual gap + 1 day	Often works out at 4–5 days
Tanker delivery	Days between deliveries + 2–3	Deliveries run late exactly when demand is highest
Own borehole or well with a pump	1–3 days	Power cuts and pump repairs
Rainwater only, off-grid	Your longest dry spell	Commonly 30–90 days or more

Size for the longest gap you normally see, not the average. If water usually comes every second day but stretches to four days each summer, four is your number. The tank earns its keep in that week.

For region-specific supply patterns, see our sizing guides for homes in Pakistan and homes in India. If three days is your target, how much water storage you need for 3 days of backup works through that case on its own.

Step 3: Add the buffer and round up

The × 1.2 at the end of the formula is a 20% buffer, and 15–25% is the usual range for homes. It pays for three things at once:

• Busy days. Guests, a heatwave, a big laundry day.
• Water the tank cannot deliver. The outlet sits a little above the floor so sediment stays behind, and the float valve shuts off below the brim. A tank labelled 1,000 litres gives you somewhat less than that at the tap.
• Error in your estimate. Both the usage figure and the backup days are approximations.

One buffer is enough. Adding a separate 10% for dead volume on top of the 20% counts the same water twice.

Then round up to the next size you can buy. Common sizes are 500, 750, 1,000, 1,500, 2,000, 3,000 and 5,000 litres, though the range differs by maker and country. Our comparison of 1,000 L vs 2,000 L vs 5,000 L tanks shows what each size suits.

Round up one size, not three. Every extra 1,000 litres is another tonne on the structure, and water that sits for many days loses the chlorine it arrived with. If stored water already smells or tastes off, read why water tastes or smells bad after storage before buying bigger.

Worked example: family of four, water on alternate days

Two adults and two teenagers. Mains water arrives every second day for about two hours, and now and then a turn is skipped.

1. Daily use. Last month’s bill shows 16.2 m³. 16,200 ÷ 30 ÷ 4 = 135 L per person, or 540 L a day for the household.
2. Backup days. The normal gap is 2 days. One skipped turn makes it 3, so they plan for 3.
3. Base volume. 4 × 135 × 3 = 1,620 L.
4. Buffer. 1,620 × 1.2 = 1,944 L.
5. Round up. The next size on sale is 2,000 L (about 530 US gallons).

Result: a 2,000-litre tank. Full, it holds about 3.7 days of water at 540 L a day. It also weighs just over 2 tonnes, and it has to refill inside a two-hour window, which means the supply or pump must deliver about 17 L/min. Those two facts decide whether 2,000 litres on the roof is realistic, and they are the checks further down.

To run the same steps with your own numbers, use the Water Tank Size for Home Calculator. Its default buffer is the same 20%, so the result will match this method.

Tank size chart by household size

Minimum tank size in litres at 135 L per person per day with the 20% buffer included. Round each figure up to the next size on sale.

People	1 day	2 days	3 days	7 days
2	324 L	648 L	972 L	2,268 L
3	486 L	972 L	1,458 L	3,402 L
4	648 L	1,296 L	1,944 L	4,536 L
5	810 L	1,620 L	2,430 L	5,670 L
6	972 L	1,944 L	2,916 L	6,804 L
8	1,296 L	2,592 L	3,888 L	9,072 L

For the four-person row in more depth, including low-use and high-use households, see what size water tank a family of 4 needs.

Three checks before you buy

The formula tells you how much water you need. It does not tell you whether that much water can sit where you plan to put it, or whether it will fill in time.

1. Can the roof carry it?

Water weighs 1 kg per litre, so a full 1,000 L tank is about 1,030 kg with its shell. On a 1.2 m × 1.0 m base that is roughly 860 kg/m².

Now compare that with what roofs are designed for. India’s IS 875 (Part 2) sets 1.5 kN/m², about 150 kg/m², for a flat roof with access. The US standard ASCE 7 uses 20 psf, about 98 kg/m², for ordinary roofs. Both codes state that these figures exclude special concentrated loads, and a water tank is one. A full tank presses down at several times the roof’s general design load.

That is why rooftop tanks belong over beams, columns or load-bearing walls, on a stand that carries the weight to them, and why a structural engineer should confirm the position. No rule of thumb says a slab “takes up to 2,000 litres”.

Run your tank through the Rooftop Load Bearing Calculator first, and read will my roof hold a full water tank for the warning signs in older buildings. If the load is too high, two smaller tanks in different spots or a ground-level tank solve it.

2. Will it refill inside your supply window?

A bigger tank only helps if it gets full. The fill rate you need is simple division:

Required fill rate (L/min) = Tank volume (L) ÷ Supply window (minutes)
2,000 L in a 2-hour window: 2,000 ÷ 120 = about 17 L/min.

Use the flow that actually reaches the tank, not the figure on the pump’s nameplate. Lift height and pipe friction take a real share of it, and the honest test is to time how long a bucket of known size takes to fill at the tank inlet. Enter that in the Tank Refill Time Calculator.

If the pump falls short, the usual cause is head. How pump head works explains it, and why a tank takes so long to refill covers the other suspects.

3. Will the pressure be enough?

Tank size has nothing to do with pressure. In a gravity-fed system, pressure comes only from the height of the water surface above the tap: about 0.1 bar (1.42 psi) for every metre. A 500 L tank and a 5,000 L tank at the same height give the same shower.

So if weak pressure is the real complaint, a bigger tank will not fix it. Check the Minimum Tank Height for Shower Pressure Calculator, or see why shower pressure is low even with a full rooftop tank.

Should the tank be overhead or underground?

Often the answer is both. An overhead tank feeds the house by gravity and keeps working in a power cut, but the structure limits its size. An underground or ground-level tank can be far larger and puts no load on the roof, but every litre has to be pumped.

Where mains pressure is too weak to reach the roof, the standard arrangement is a ground-level sump that fills from the mains and a pump that lifts water to a smaller overhead tank. Size the total storage with the formula above, then split it. A common split is about one day’s use overhead and the rest in the sump.

Overhead vs underground water tanks compares the two in full. The Underground vs Rooftop Tank Cost Calculator totals upfront and running costs for each from your own quotes.

Sizing mistakes to avoid

• Using a survival figure for a household tank. 15 L and 50 L per person are humanitarian and health thresholds. A piped home draws two to three times the 50 L level.
• Sizing for the average gap. The tank is tested by the longest gap, and that usually arrives in the hottest weeks, when use is also at its highest.
• Buying first and checking the roof later. The structure sets the ceiling on a rooftop tank. Check it before you choose a size.
• Upsizing the tank and keeping the old pump. Twice the volume takes twice as long to fill. If the window is short, the new tank never fills.
• Reading the label as usable water. Plan on the buffer covering the layer below the outlet. If you know your tank’s dead volume, the How Long Will My Tank Last Calculator has a field for it.
• Sizing for today’s household only. A tank lasts 15 years or more. Children grow up and families grow.

Already own a tank and suspect it is the wrong one? How to tell if your water tank is the wrong size lists the signs, and my water runs out in 2 days covers the most common one.

More calculators for this job

• Apartment Water Tank Size Calculator: shared storage for a whole building, from flats and residents per flat.
• Water Tank Weight Calculator: full weight of any tank, shell included.
• Rainwater Harvesting Calculator: how much your roof can collect in a year, to see what rain can take off the mains.
• Emergency Water Storage Calculator: a separate drinking-water reserve for disasters.

Frequently asked questions

What size water tank do I need for a family of 4?

At 135 litres per person per day with a 20% buffer, a family of 4 needs 648 L for 1 day of backup, 1,296 L for 2 days and 1,944 L for 3 days. In tank sizes that is 750 L, 1,500 L and 2,000 L. Pick the row that matches your longest normal gap in supply.

How long will a 1,000-litre tank last a family of 4?

At 135 litres each, a family of 4 uses 540 L a day, so a full 1,000 L tank lasts about 1.85 days, or roughly 44 hours. At 100 L each it lasts 2.5 days, and at 200 L each only 1.25 days. Allow a little less in practice for the water below the outlet. Our guide on how to calculate how long a water tank will last has the full method.

Is a 500-litre tank enough for a house?

For one or two people with water every day, yes: two people at 135 L use 270 L a day, so 500 L covers a full day with room to spare. For a family of four it is not enough. They use about 540 L a day, which is more than the tank holds.

How do I calculate water tank size in litres?

Multiply people × litres per person per day × backup days × 1.2. For example, 5 people × 100 L × 3 days × 1.2 = 1,800 L, which rounds up to a 2,000 L tank. Divide litres by 3.785 for US gallons: 2,000 L is about 528 US gallons.

How many days of water should a home tank hold?

Enough to cover the longest gap you normally see between refills. That is about 1 day on reliable mains, 2–3 days where water comes on alternate days, and the delivery interval plus 2–3 days if you rely on tankers. This is separate from emergency drinking water: Ready.gov advises at least 1 gallon per person per day for several days, which is a far smaller volume.

Does a bigger tank give better water pressure?

No. Gravity pressure depends on how high the water surface is above the tap, about 0.1 bar per metre, not on how much water the tank holds. To improve pressure, raise the tank or add a booster pump.

What size tank does a home with no mains supply need?

Size it for the longest stretch between refills, whether that is a tanker interval or a dry season. A family of 4 using a careful 80 L each through a 30-day gap needs 4 × 80 × 30 × 1.2 = 11,520 L, usually three 5,000 L tanks. The Off-Grid Water Storage Calculator works out the volume and the number of tanks for any gap.

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Sources

• World Health Organization, Domestic water quantity, service level and health, 2nd edition (2020). Service levels: about 50 L per person per day as intermediate access, 100 L or more as optimal.
• Sphere Association, The Sphere Handbook (2018). Minimum 15 L per person per day in humanitarian response.
• Bureau of Indian Standards, IS 1172:1993, Code of Basic Requirements for Water Supply, Drainage and Sanitation. 135 to 200 L per head per day for homes with full flushing systems, of which 45 L may be taken for flushing.
• Defra, Outcome Indicator Framework, E8: Efficient use of water. Per capita household consumption in England, 139.47 L per person per day in 2023/24.
• US EPA WaterSense, Statistics and Facts. 82 gallons per person per day at home. Fixture limits from WaterSense showerheads and residential toilets.
• ENERGY STAR, Clothes Washers. 14 gallons per load for certified full-size washers against 20 gallons for a standard machine.
• Ready.gov (FEMA), Water. At least 1 gallon per person per day for several days; needs can double in very hot temperatures.
• Bureau of Indian Standards, IS 875 (Part 2):1987, Imposed Loads, Table 2. 1.5 kN/m² for flat roofs with access, 0.75 kN/m² without.
• American Society of Civil Engineers, ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. 20 psf minimum roof live load for ordinary roofs.
• US Geological Survey, Water Density. About 1 kg per litre (8.34 lb per US gallon).