Enter the number of flats, residents per floor, and daily usage pattern to get a precise recommended tank capacity — daily demand, backup storage, and total size in litres.
Fill in your building's details — results appear instantly
Building Details
Flats
units
Floors
lvls
Per Flat
ppl
Daily Usage Level
Backup Days Required
2 days
1 day234567 days
⚠ Please enter valid numbers (greater than zero) for flats, floors, and residents per flat.
WaterTankCalculator.com — Apartment Tank Size Results
Your Results
Recommended Tank Capacity
—
Litres (L)
Total Residents
— ppl
Daily Demand
— L/day
Usage Level
— L/pp
Backup Days
— days
Base Storage
— L
+ 15% Buffer
— L
US Gallons
— gal
Full Load Weight
— kg
Per-Flat Demand
— L/day
Building Storage Level
📖 How To Use
How to Use This Calculator
Sizing a water tank for an apartment building takes 30 seconds with the right inputs:
Enter the number of flats
Count the total residential units in the building — not bedrooms, not floors, just distinct apartments or flats. Include all units, even rarely occupied ones; you're sizing for peak occupancy.
Enter the number of floors
Floors don't change the volume, but they set how high the water must be lifted to a roof tank — the results note estimates that lift at about 3 m per floor, which you can take to the pump calculator. Taller buildings usually pair an underground tank with a smaller overhead tank.
Enter residents per flat
Use the real average for the building if you know it. Typical household sizes vary widely — around 4–6 people in much of South Asia, and about 2–2.5 in many European and North American cities. Sizing for full occupancy is safer than sizing for today's.
Select a usage level
Basic (100 L/person/day) suits water-careful homes. Standard (150 L) is close to the 150 L per person that India's CPHEEO manual uses for metropolitan cities with sewerage (135 L for other sewered cities). Comfort (200 L) allows longer showers and more laundry; Luxury (250 L) covers dishwashers, frequent laundry and garden taps. If you know the building's actual use from its bulk meter, type it into "Or enter your own".
Set your backup days
This is how many days of full consumption the tank must cover without a refill. Match it to the longest normal gap between supplies, plus any outages you want to ride out — for example 2 days where the mains supplies on alternate days.
Tip: The recommended size already includes a 15% margin above the raw storage. Don't size a tank to the exact demand — the water below the outlet can't be used, tanks are not filled to the brim, and busy days use more.
📐 The Formula
Apartment Water Tank Size Formula
The calculation is straightforward but the inputs matter enormously:
Total Residents = Flats × Residents per Flat
Daily Demand (L) = Total Residents × Usage Rate (L/person/day)
Base Storage (L) = Daily Demand × Backup Days Recommended Tank = Base Storage × 1.15
The 1.15 multiplier adds a 15% margin, a common rule of thumb. Where supply is unreliable, many designers use 1.25 or more — or simply add backup days.
Usage Rate Reference Table
Usage Level
L / Person / Day
Typical Context
Benchmark
Basic
100 L
Water-careful homes, water-scarce regions
Above the WHO 50 L "intermediate access" level
Standard
150 L
Typical city apartment
CPHEEO (India): 135 L sewered cities, 150 L metros
Comfort
200 L
Longer showers, more laundry
Planning value
Luxury
250 L
High-end residential, full appliance use
Planning value; US homes average ~310 L (US EPA)
Backup Storage by Supply Reliability
These are common planning choices, not code requirements — match the days to the longest gap you actually see.
Supply Situation
Backup Days
Examples
Highly reliable mains supply
1 day
24-hour supply with rare cuts
Generally reliable, occasional outage
2 days
Daily supply, occasional cuts
Regular scheduled rationing
3–4 days
Supply on alternate days or a few hours a day
Unreliable / frequent outages
5–7 days
Irregular supply or tanker deliveries
📏 Quick Reference
Typical Apartment Building Tank Sizes
Based on standard usage (150 L/person/day), 4 residents per flat, and 2 days backup storage with 15% buffer:
Building Size
Flats
Total Residents
Daily Demand
Recommended Tank
Small block
4
16
2,400 L
~5,500 L
Small block
8
32
4,800 L
~11,000 L
Mid-rise
16
64
9,600 L
~22,100 L
Mid-rise
24
96
14,400 L
~33,100 L
High-rise
40
160
24,000 L
~55,200 L
Large complex
80
320
48,000 L
~110,400 L
Large complex
120
480
72,000 L
~165,600 L
Actual requirements depend on local regulations, roof load limits, and supply conditions. Always verify with a licensed plumber or civil engineer before procurement.
🏢
Rooftop vs. Underground
Rooftop tanks rely on gravity for pressure — simple but structurally demanding. Underground tanks need a pump but avoid roof load. Many multi-storey buildings combine the two: an underground tank holds the main reserve and a pump fills a smaller overhead tank.
⚖️
Roof Load Reality
1,000 litres of water weighs about 1,000 kg. A 10,000-litre rooftop tank adds 10 tonnes, plus the tank itself. Get a structural engineer to check the roof before installation.
🔧
Split Tanks
Large requirements are often met with multiple tanks in parallel. This also enables maintenance shutdowns without completely cutting off supply to the building.
🏠 Use Cases
When Do You Need This Calculator?
New Building Construction
Architects and developers use this during design-stage planning to spec the right tank size and check whether the rooftop structure can bear the load, or whether an underground cistern is needed. Getting this wrong at design stage is expensive to fix later.
Replacing an Undersized Existing Tank
If your building is running out of water before daily supply arrives, this calculator helps you quantify the deficit. Compare the recommended output against your current tank size to understand how much extra capacity you need to add.
Post-Construction Renovation
Building managers who have converted additional flats, added floors, or seen occupancy increase above original design specs can use this to check whether the original tank is still adequate for the current resident count.
Emergency Preparedness Planning
Adjust the backup days slider to see how much storage you'd need to survive a 3-day, 5-day, or 7-day supply disruption — relevant for buildings in areas prone to water supply outages, natural disasters, or civil emergencies.
Housing Societies & Facility Managers
Use the output as a benchmark when requesting quotes from tank suppliers. The recommended tank size gives you a starting point; factor in your local building codes and any regulatory minimum storage requirements before finalising the spec.
🧮 Worked Example
Worked example: 12 flats on 4 floors
A 4-storey block has 12 flats with about 5 people each. The city supplies water on alternate days, so the building wants 2 days of storage, at 135 litres per person per day (the CPHEEO figure for sewered cities).
Residents: 12 × 5 = 60 people.
Daily demand: 60 × 135 = 8,100 L/day (675 L per flat).
Storage: 8,100 × 2 days = 16,200 L, + 15% = 18,630 L (about 4,920 US gallons).
Layout: 18.6 tonnes of water is a lot for a roof. A common split is an underground tank holding most of it, with a pump lifting about 12–15 m (4 floors at roughly 3 m each, plus the tank stand) into a smaller overhead tank.
❓ FAQ
Frequently Asked Questions
How do I calculate water tank size for an apartment building?
Multiply the total number of residents by the daily water use per person (for example 135–150 litres, the CPHEEO figures for Indian cities with sewerage), then multiply by your desired backup days. Add a minimum 15% safety buffer on top. The formula: Tank Size = (Residents × Daily Usage × Backup Days) × 1.15. This calculator does all of that for you automatically.
What is the standard water tank size for a 10-flat apartment building?
With 4 residents per flat (40 total), standard usage of 150 L/person/day, and 2 backup days, the recommended size is roughly 13,800 litres (including 15% buffer). In practice, you'd install two 7,000-litre tanks or one 15,000-litre tank depending on available space and structural constraints.
How many litres of water does an apartment building use per day?
It depends heavily on the number of residents and their habits. Design figures for city apartments are often 135–200 litres per person per day; the building's bulk water meter gives the real figure. A 20-flat building with 4 residents each (80 people) at 150 L/day consumes around 12,000 litres per day.
Should I use a rooftop tank or an underground sump for my apartment?
For low-rise buildings (up to 4–5 floors), a rooftop tank is the simplest and cheapest option — gravity feeds water pressure without a pump. For taller buildings, a combined system is standard: a large underground sump stores the main reserve and a pump fills a smaller overhead tank, which gravity-feeds the building. Large requirements (say 10,000+ litres, which is 10 tonnes of water) often go underground regardless of height, because a roof must be designed to carry that weight — what a given roof can hold safely is for a structural engineer to confirm.
What is the recommended water storage per person per day?
The WHO links about 50 litres per person per day with low health risk and 100 litres or more with optimal access. For design, India's CPHEEO manual uses 135 litres per person per day for cities with sewerage and 150 litres for metropolitan cities; many comfort-standard apartments use 150–200 litres. US homes average about 310 litres, including garden watering. Use the usage level selector in this calculator to match your building's real-world consumption pattern.
How many backup days should I size my apartment water tank for?
Match it to your supply. With reliable 24-hour supply, 1 day is common; with supply on alternate days, at least 2; with irregular supply or tanker deliveries, the longest gap you expect between refills. Go bigger if outages are common — but very large tanks hold water longer, so plan regular cleaning.
Can I split one large tank into multiple smaller ones?
Yes, and for large buildings it's often the better approach. Multiple tanks in parallel give you redundancy — one can be taken offline for cleaning or repair without cutting off the whole building. They also distribute roof load more evenly. Use the recommended litres output from this calculator as your total target and split it across as many tanks as makes structural and logistical sense.
Does the number of floors affect how big the water tank needs to be?
The number of floors doesn't directly affect storage volume — that's determined by residents and usage. However, floors do affect pressure requirements and pump sizing. A rooftop tank gives each floor about 0.1 bar (1.4 psi) of pressure per metre of height above it, so the top floors get very little — they may need a booster pump or a taller tank stand — while the lowest floors of a tall building may need pressure-reducing valves. Use our Minimum Tank Height for Shower Pressure calculator for pressure planning.
✅ Sources & Review
Sources and review
The sizing formula is simple arithmetic; the per-person design figures it is compared with come from these references:
CPHEEO (Government of India) — Manual on Water Supply and Treatment: 70 L per person per day for towns with piped supply but no sewerage, 135 L for cities with sewerage, 150 L for metropolitan cities with sewerage.
US EPA WaterSense — Statistics and Facts: 82 gallons (about 310 L) per person per day at home.
Water pressure from height: 1 m of water ≈ 0.098 bar ≈ 1.42 psi; 1 US gallon = 3.785 L (NIST SP 811).
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
