Calculate the capacity of a rectangular or cylindrical water tank in litres and US gallons.
When you choose feet, each dimension is multiplied by 0.3048 to convert it to metres, the volume is found in cubic metres, and that is multiplied by 1,000 to give litres. The answer is rounded to the nearest whole litre.
Handy rules for checking: 1 cubic foot holds about 28.3 litres, and 1 cubic metre holds exactly 1,000 litres.
The calculator gives the geometric volume up to the height you enter. In a built tank, the water never reaches the very top: it stops at the overflow pipe, and the outlet is usually set a little above the floor so sediment is not drawn into the pipes.
For a realistic figure, enter the depth between the outlet and the overflow level instead of the full internal height. In the 5 × 4 ft sump above, losing 6 inches at the top for freeboard reduces the capacity from 3,398 to about 3,115 litres.
Always measure inside dimensions. Concrete sumps often have walls 150 to 230 mm thick, so outside measurements can overstate the volume considerably.
Start from daily consumption. Indian water supply guidelines commonly use around 135 litres per person per day for homes with full flushing, although actual use varies widely. A family of four would use about 540 litres a day on that basis.
Where municipal supply comes on alternate days or at fixed hours, the sump is often sized to hold one to two days of use (about 1,080 litres for that family at two days), with the overhead tank holding roughly a day's supply. Check local bye-laws for any minimum storage rules, and size a fire-fighting reserve separately in apartment buildings.
Plastic overhead tanks are sold in standard sizes such as 500, 1,000 and 2,000 litres. Round your required capacity up to the next standard size rather than down.