The formulas

Rectangular tank: litres = length × width × height (in metres) × 1000
Round (cylindrical) tank: litres = π × radius² × height (in metres) × 1000
In feet: litres = cubic feet × 28.32

One cubic metre holds 1,000 litres. One cubic foot holds about 28.32 litres. Always use inside dimensions.

EXAMPLE · RECTANGULAR

Inside 6 × 4 ft, water depth 4 ft: 6 × 4 × 4 = 96 cft × 28.32 = 2,718 litres

EXAMPLE · ROUND

Inside diameter 1.2 m (radius 0.6 m), water height 1.5 m: π × 0.6² × 1.5 = 1.696 m³ = 1,696 litres

Total volume vs usable water

Not all the volume is usable. The space above the overflow pipe and the water below the outlet cannot be drawn. Allow for this when comparing a tank's size with a household's need.

Sizing for a household

Daily need (litres) = people × litres per person per day

135 litres per person per day is a planning figure widely used in India for urban homes with flushing (it appears in the CPHEEO water-supply manual). Actual use varies a great deal. Storage is usually planned for one or more days of use, depending on how reliable the supply is.

EXAMPLE

Family of 4 × 135 L = 540 L a day. A tank with 2,400 L of usable water lasts about 4.4 days.

Tool

The tank capacity calculator works for rectangular and round tanks in feet, inches or metres, and shows how many days the water lasts.