About this calculator
The calculator also shows how many days the water lasts a family of four.
How to use it
- Choose the shape and units.
- Enter the inside size.
- Enter the water depth.
The formula
Rectangular: litres = length × width × depth × 1,000 (m); round: π × d² ÷ 4 × depth × 1,000
- 1 m³
- 1,000 litres
- 1 cft
- 28.3 litres
Worked example
A 6 × 5 ft sump, 5 ft deep
- Volume = 150 cft = 4.25 m³.
- Capacity = 4,248 litres, about 8 days for four people at 135 litres each.
What the result means
Size a home tank for at least one to two days of use.
Assumptions
- Inside measurements.
Limitations
- Walls and slope at the bottom reduce real capacity slightly.
Frequently asked questions
How many litres in a 1,000 cft tank?
About 28,317 litres.
How much water does a family use?
Around 135 litres per person a day in cities.
How do I measure a round tank?
Measure the inside diameter and water depth.
Last reviewed on 3 October 2026. Found a mistake? Tell us.