Water Tank Size Calculator
Size a household water storage tank: daily use by people and habits, the rooftop tank for one day, the cistern or ground tank for several days without supply, and the next standard tank size in litres or gallons.
How it works
daily use D = people × litres per person per day · rooftop tank ≥ D (one day), rounded up to a standard size · cistern ≥ D × days without supply, rounded up · autonomy = (tank + cistern) ÷ D
Homes with intermittent mains supply store water twice: a cistern or ground tank that refills whenever the network delivers, and a rooftop or elevated tank that feeds the house by gravity. Design manuals size the elevated tank for about one day of use and the cistern for the days the supply can fail, both from a per-person daily allowance — 100 to 250 litres depending on climate, fixtures and garden. Rounding up to the next commercial tank (450, 600, 750, 1,100 L… for rooftop tanks; 1,200, 2,800, 5,000 L… for cisterns) gives what to buy. Add fire or irrigation reserves separately if your local code requires them.
Worked example
A family of 4 using a typical 150 L each needs 600 L a day. The rooftop tank should hold one day — a standard 600 L tank; the cistern for 3 days must hold 1,800 L, so the next size is 2,800 L. With both full the house has (600 + 2,800) ÷ 600 = 5.7 days of water.
Sources: WHO — Domestic water quantity, service level and health · CONAGUA — Manual de Agua Potable, Alcantarillado y Saneamiento
Frequently asked questions
What size water tank does a family of 4 need?
At 150 L per person a day, 600 L a day: a 600–750 L rooftop tank and a cistern of 1,800 L or more for three days — in practice the 2,800 L size.
How much water does one person use per day?
Between 100 and 250 L at home depending on showers, toilets, washing machine and garden. The WHO considers 50–100 L the minimum for health and hygiene; many building codes use 150–200 L per person for design.
Is a bigger tank always better?
Not for drinking water: water that sits for weeks loses residual chlorine. Size for the days without supply you actually face, clean the tank every 6–12 months and keep it covered and opaque.
Should the cistern be bigger than the rooftop tank?
Yes. The rooftop tank only buffers a day of gravity supply; the cistern bridges the supply gaps and lets a pump refill the tank, so it is usually 2–4 times larger.
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This calculator is for educational purposes. Double-check important results before acting on them.