Tell us about your household — we'll size the right tank
Household details
People
ppl
Backup Days
days
Daily water usage per person
Usage / Person / Day
Safety %
%
Quick Usage Presets (per person / day)
⚠ Please enter valid values for all fields (people ≥ 1, usage > 0, backup days ≥ 1).
WaterTankCalculator.com — Home Tank Size Results
Recommended Tank Size
Minimum Recommended Capacity
—
Litres (L)
US Gallons
— gal
UK/Imperial Gallons
— gal
Daily Household Need
— L/day
Base Volume
— L
Safety Buffer Added
— L
Full Weight (Water)
— kg
Household Water Needs Visual
📖 How To Use
How to Use This Water Tank Size Calculator
Getting the right home water tank size takes under a minute:
Enter the number of people
Count everyone who regularly uses water in your home — family members, live-in staff, and regular guests. If you expect the household to grow, size for the future.
Set your daily usage per person
Use the presets or type your own number. Many piped urban homes use 100–200 litres per person per day; water-careful or rural homes 50–100 L; US homes average about 310 L including garden watering. The most accurate figure is your own: divide a month's metered use by 30 days and by the number of people.
Choose your backup days
This is how many days the tank should supply the house with no refill. Match it to your longest normal gap in supply — for example 2 days if water comes every other day — plus any outages you want to ride out. Homes that rely on tanker deliveries or rain plan for the longest gap between refills.
Set a safety buffer (recommended: 20%)
A margin covers guests, hot-weather peaks, and the water below the outlet that the tank cannot deliver. 15–25% is a common rule of thumb. Set it to 0 to see the bare minimum.
Hit Calculate
Your recommended minimum capacity appears in litres, US gallons and UK gallons, with the next standard tank size, how long that tank lasts when full, and the working.
Tip: Always buy the next standard size up from your calculated minimum. If the calculator says 820 litres, buy a 1,000-litre tank — commercial tanks come in standard increments and the extra headroom costs little but saves a lot.
📐 The Formula
Water Tank Size Formula for Homes
The calculation uses three inputs to find the minimum required storage, then adds a safety margin:
Base Volume = People × Usage per Person × Backup Days
Recommended Size = Base Volume × (1 + Safety Buffer ÷ 100)
Example: 4 people × 150 L/day × 2 days = 1,200 L base
With 20% buffer: 1,200 × 1.20 = 1,440 L recommended
The safety margin covers usage spikes, the water below the outlet that the tank cannot deliver (often a few percent of capacity), and gaps in supply a little longer than planned. A 15–25% margin is a common rule of thumb.
Standard Usage Benchmarks
Usage Level
Litres / Person / Day
Typical Context
Basis
Survival minimum
15 – 20 L
Emergency / disaster
Sphere 15 L; WHO "basic access" ~20 L
Basic needs
50 L
Water-scarce homes
WHO "intermediate access" ~50 L
Low comfort
80 – 100 L
Water-conscious urban
UN/WHO 50–100 L basic-needs range
Standard comfort
130 – 170 L
Typical urban household
Planning range
High usage
200 – 250 L
Large homes, gardens
Planning range
Luxury / villa
300 – 400 L
Pools, irrigation, staff
Planning range; US homes average ~310 L (US EPA)
Common Tank Sizes vs Household Match
Days of backup each tank size gives at 150 L per person per day with the 20% safety margin kept in reserve (days = tank ÷ (people × 150 × 1.2)). The tank shell adds weight on top of the water.
Tank Size
2 People
4 People
6 People
Water Weight (Full)
500 L
1.4 days
0.7 days
0.5 days
500 kg
1,000 L
2.8 days
1.4 days
0.9 days
1,000 kg
1,500 L
4.2 days
2.1 days
1.4 days
1,500 kg
2,000 L
5.6 days
2.8 days
1.9 days
2,000 kg
3,000 L
8.3 days
4.2 days
2.8 days
3,000 kg
5,000 L
13.9 days
6.9 days
4.6 days
5,000 kg
10,000 L
27.8 days
13.9 days
9.3 days
10,000 kg
🏠 Use Cases
When Do You Need a Home Water Tank?
⚡
Unreliable Supply
Frequent cuts, low pressure, or seasonal dry-outs make storage essential for uninterrupted daily life.
🌧️
Rainwater Harvesting
Capture and store roof runoff during wet seasons to supplement supply during dry months.
🏔️
Off-Grid Living
No mains connection means the tank must bridge the longest gap between deliveries, pumping or rain — often a week or more.
🚨
Emergency Preparedness
Disasters and contamination events can cut supply for days. Ready.gov advises storing at least 1 gallon of drinking water per person per day for at least 3 days.
💧
Pressure Boosting
A rooftop tank provides gravity-fed pressure even when mains pressure drops below usable levels.
💸
Cost Savings
Fill during off-peak hours or cheaper tariff periods and reduce reliance on expensive tanker deliveries.
🧮 Worked Example
Worked example: water every other day for a family of five
A family of five gets mains water every second day. Their bills show about 18,000 litres a month, which is 18,000 ÷ 30 ÷ 5 = 120 L per person per day. They want the tank to cover a 2-day gap, with a 20% margin.
Daily household need: 5 × 120 = 600 L/day.
Base volume: 600 × 2 days = 1,200 L.
With 20% margin: 1,200 × 1.20 = 1,440 L (about 380 US gallons).
Tank to buy: the next standard size, 1,500 L. Full, it lasts 1,500 ÷ 600 = 2.5 days, and holds 1,500 kg of water — check the roof or stand before installing it high up.
❓ FAQ
Frequently Asked Questions
How do I calculate the right water tank size for my home?
Multiply the number of people in your household by daily water usage per person (typically 100–200 litres), then multiply by the number of days of backup storage you want. Add 15–25% as a safety margin. For example, 4 people × 150 L × 2 days × 1.20 = 1,440 litres minimum.
What size water tank does a family of 4 need?
For a family of 4 with 2 days of backup at 150 L/person/day, the minimum is around 1,440 litres including a 20% buffer. A 1,500 L or 2,000 L tank gives that with some headroom. For 3 days the same family needs about 2,160 litres (a 2,500 L tank); for 5 days, about 3,600 litres.
How many litres of water does a person use per day at home?
The WHO links about 50 litres per person per day with low health risk and 100 litres or more with optimal access. Many piped urban homes use 100–200 litres per person, covering drinking, cooking, bathing, toilets and laundry; US homes average about 310 litres (82 gallons) per person, according to US EPA WaterSense, with more than 30% used outdoors. Homes with large gardens or pools can use far more.
How many days of backup storage should my home tank provide?
It depends on your supply. Match the backup days to the longest normal gap — for example 2 days where water comes every other day — and add more if outages are common. Homes relying on tanker deliveries or rain need to cover the longest gap between refills. Separately, Ready.gov and the CDC advise every household to keep at least 1 gallon of drinking water per person per day for at least 3 days, which is a much smaller amount than a whole-house supply.
Why add a safety buffer when sizing a home water tank?
A safety margin covers peak days (guests, heat waves), the water below the outlet that the tank cannot deliver, errors in your usage estimate, and supply gaps a little longer than planned. 15–25% is a common rule of thumb for homes. (Drinking-water tanks should always be covered, so evaporation is not a real factor.)
Is it better to have one large tank or multiple smaller tanks?
One large tank is usually more cost-effective and easier to maintain. Multiple smaller tanks offer flexibility — easier to transport, can be staged over time, and reduce the structural load at any single point. For rooftop installations where structural load is a concern, multiple smaller tanks distributed across the roof are often the safer choice. Use our Rooftop Load Bearing Calculator to check your roof capacity first.
What's the difference between a rooftop tank and an underground tank for home use?
Rooftop tanks deliver water by gravity — each metre of height gives about 0.1 bar (1.42 psi) — but often need a pump to fill them, and their weight is a critical structural concern. Underground tanks can hold much larger volumes (5,000–50,000+ litres), aren't exposed to heat or UV, but require a pump to distribute water. For most urban homes, a rooftop polyethylene tank in the 1,000–3,000 L range is the most practical solution.
How heavy is a full home water tank — can my roof handle it?
Water weighs 1 kg per litre. A full 1,000 L polyethylene tank weighs roughly 1,020–1,050 kg including the shell. Roofs are commonly designed for live loads of only about 1–2 kN/m² (roughly 100–200 kg/m²), depending on the building code and how the roof is used. A 1,000 L tank on a 1.2 m × 1.0 m base puts about 870 kg/m² on that area — several times more. Always use a load-spreading frame and consult a structural engineer before installation. Use our Rooftop Load Bearing Calculator for exact figures.
✅ Sources & Review
Sources and review
The sizing formula is simple arithmetic; the usage benchmarks it is checked against come from these references:
US EPA WaterSense — Statistics and Facts: 82 gallons (about 310 L) per person per day at home; outdoor use over 30% on average.
Ready.gov (FEMA) — Water: at least 1 gallon per person per day for at least 3 days for emergencies.
Water weighs about 1 kg per litre (0.998 kg at 20 °C); 1 US gallon = 3.785 L and 1 UK gallon = 4.546 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.
