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Water Tank Calculator
Two tools in one: find the tank size your home needs from the number of people, or work out any tank's capacity in litres from its dimensions — rectangular or cylindrical.
What do you want?
Recommended tank—
Daily requirement—
Suggested overhead tank—
Suggested underground sump—
How it's calculated
For the size you need, the number of people times litres per person per day times the days of storage gives the total. A common split is roughly a third in the overhead tank and the rest in an underground sump. For capacity, one cubic metre holds 1,000 litres: a rectangular tank is length × width × height, and a cylindrical tank is π × radius² × height.
Daily need = people × litres/person × days. 1 m³ = 1,000 litres. Add a margin for guests and supply gaps.
Frequently asked questions
Overhead vs underground?
The underground sump stores the bulk from the mains; a pump lifts water to a smaller overhead tank for pressure.
Is 135 litres too much?
It's the standard domestic figure with flushing; drinking and cooking alone are far less. Lower it if your supply is tight.
What's a 1000 litre tank in feet?
About 1 m³ — roughly 1 m on each side, or a slightly taller, narrower shape.