How much water does your cistern hold?
Measure the inside of the tank, not the outside, and use the depth the water reaches when the tank is full (usually up to the overflow), not the full height of the walls.
The formulas
- Rectangular tank: length × width × water depth.
- Round tank: 3.1416 × (diameter ÷ 2)² × water depth.
- 1 cubic foot holds 7.48 US gallons (6.23 imperial gallons, 28.3 litres).
- 1 cubic metre holds 1,000 litres (264 US gallons, 220 imperial gallons).
Usable water
Not all of it is usable. Only the water above your pump's stop level can be pumped to the roof. If the tank holds 4,300 gallons and the pumps stop at 25%, about 3,200 gallons are usable on each fill.
US or imperial gallons?
Both are used in the Caribbean, and they differ by about 20%. An imperial gallon is 4.55 litres; a US gallon is 3.79. Check which one your water meter, your pump's data plate and your water bill use before comparing numbers.
Why gallons per inch matters
Once you know how many gallons one inch (or one centimetre) of depth holds, any change in level becomes a volume. If the cistern rises 3 inches an hour while the city supply is on, and each inch is 400 gallons, the city is giving you 1,200 gallons an hour. If it falls 2 inches an hour overnight with the pumps off, you're losing 800 gallons an hour somewhere.
A level sensor does this sum every few minutes. See how it works.
Want this done for you?
Sensors in your tanks, pumps that run themselves, alerts when something needs a person.
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