Sizing a tank
A water tank is just a box of water, so its capacity comes straight from its inside measurements. What matters more is whether that capacity carries the household through a day without supply — which is why this calculator also works out how long the tank lasts.
Formula
Volume in cft = length × width × depth of waterLitres = cft × 28.3168·1 m³ = 1,000 litresDays of supply = litres ÷ (people × daily use)
Worked example
A tank 6 × 4 × 4 feet holds 96 cft, which is 2,718 litres or 718 US gallons. For five people using 135 litres each a day, that is just over four days of water.
How to use the tank calculator
- Measure inside the tank, not outside — the walls do not hold water.
- Use the water depth, which is usually a few inches below the top of the tank.
- Set people and daily use. 135 litres per person a day is the common design figure for a household with piped supply.
Practical notes
- Plan at least one day of storage, and two if your supply is unreliable. Overhead and underground tanks together are often sized for two or three days.
- Water weighs about 1 kg per litre. A 2,700 litre tank puts nearly 2.7 tonnes on the roof, so the slab has to be designed for it.
- For a cylindrical tank, use π × radius² × depth for the volume instead.
Frequently asked questions
How many litres is a 6x4x4 feet tank?
About 2,718 litres. The volume is 96 cubic feet, and each cubic foot holds 28.3 litres.
What size water tank does a family need?
Allow around 135 litres per person a day. For five people that is 675 litres a day, so a 1,000 litre tank covers a day and a 2,000 litre tank covers three.
How do I convert cubic feet to litres?
Multiply cubic feet by 28.3168. One cubic metre is 1,000 litres, or 35.31 cubic feet.