Formulas
| Tank | Formula | Example |
|---|---|---|
| Box (rectangular) | Length × width × height | 5 × 4 × 4 ft = 80 cft = 2,265 L |
| Round (cylinder) | 3.1416 × (diameter ÷ 2)² × height | 4 ft across, 5 ft high = 62.83 cft = 1,779 L |
| Household need | People × litres per person per day × days | 5 × 100 × 2 = 1,000 L |
| Fill time | Litres ÷ pump flow (L/min) | 2,000 ÷ 40 = 50 min |
Examples
Rooftop concrete tank: 5 × 4 ft inside, with water up to 4 ft. That is 80 cft, about 2,265 litres (598 US gallons). A 40 L/min pump fills it in about 57 minutes.
Underground reservoir measured in metres: 2 × 1.5 × 1.2 m = 3.6 m³ = 3,600 litres.
Buying a new tank: six people at 135 litres each for two days need 1,620 litres. The next common size is 2,000 litres.
What to know in Bangladesh
- Measure the inside. Concrete tank walls are 5–10 inches thick, so outside measurements overstate capacity. Take the height up to the overflow pipe, since water never rises above it.
- Litres per person: the UN puts basic needs at 50–100 litres a day. The calculator starts at 100; city homes with flush toilets, showers and laundry may use more. Check your water meter or bill and adjust.
- Days of storage: where WASA or municipal water comes at fixed hours or stops now and then, keep one to two days in storage.
- Ready-made plastic tanks usually come in 300, 500, 750, 1,000, 1,500, 2,000, 3,000, 5,000 and 10,000 litres. Sizes differ a little by brand, so check before buying.
- Weight: a litre of water weighs about 1 kg. Have an engineer confirm the roof can carry a large tank.
Tank size by household (100 litres per person per day)
| People | 1 day | Common size | 2 days | Common size |
|---|---|---|---|---|
| 3 | 300 L | 300 L | 600 L | 750 L |
| 4 | 400 L | 500 L | 800 L | 1,000 L |
| 5 | 500 L | 500 L | 1,000 L | 1,000 L |
| 6 | 600 L | 750 L | 1,200 L | 1,500 L |
| 8 | 800 L | 1,000 L | 1,600 L | 2,000 L |
| 10 | 1,000 L | 1,000 L | 2,000 L | 2,000 L |
More: unit conversions and measuring pump flow
Why litres are easy: a litre is a 10 cm cube, so metres are the simplest unit: multiply cubic metres by 1,000. In feet, find cubic feet first and multiply by 28.32. In inches, divide cubic inches by 1,728 to get cubic feet.
Two kinds of gallon: a US gallon is 3.785 litres and an imperial (UK) gallon is 4.546 litres, so the same tank shows a bigger number in US gallons. If a seller quotes “500 gallons”, ask which kind, or talk in litres.
Diameter of a round tank: wrap a tape round the tank and divide the circumference by 3.1416. Plastic tanks narrow near the lid, so the result can run slightly high.
Measuring pump flow: hold a 20-litre bucket under the pipe that fills the tank and time it. Divide 1,200 by the seconds: 30 seconds means 40 L/min. The flow printed on a motor is usually at the lowest lift; pushing water up to a roof gives less, so the bucket test is more reliable.