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Water tank calculator
Find how much water a tank holds, in litres and gallons, from its inside size. Works for rectangular, square and round tanks.
Result
Enter your sizes to see the result.
How it’s worked out
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How to use this calculator
- Choose the tank shape: rectangular / square, or round.
- Enter the inside length, width and water height in feet, or the diameter and height for a round tank.
- Read the capacity in litres, UK gallons, US gallons, CFT and cubic metres.
The formula
Rectangular tank: volume = length × width × height. Round tank: volume = π × radius² × height, where radius = diameter ÷ 2. Litres = CFT × 28.317; UK gallons = litres ÷ 4.546; US gallons = litres ÷ 3.785.
Frequently asked questions
How many litres are in 1 cubic foot?
1 CFT holds 28.317 litres of water.
How many gallons is a 10 × 8 × 6 ft tank?
10 × 8 × 6 = 480 CFT = 13,592 litres, which is about 2,990 UK gallons or 3,591 US gallons.
Which gallon is used in Pakistan?
Tank makers and water tankers in Pakistan usually quote UK (imperial) gallons, 4.546 litres each. US gallons are 3.785 litres.
Should I use the inside or outside size?
Use the inside size, and the height of the water, not the full wall height, since the tank is never filled to the top.