US gallons and imperial gallons are not interchangeable

A gallon label is incomplete unless it identifies the system. The international US liquid gallon is 3.785411784 litres. The imperial gallon is 4.54609 litres. A tank described as holding the same number of gallons can therefore have different capacities.

A worked example

Item	Example or check
100 US liquid gal	378.5411784 L
100 imperial gal	454.609 L
1,000 L	About 264.172 US gal
1,000 L	About 219.969 imperial gal

Check the assumptions

For a rectangular tank, measure inside length and width and the intended water depth. Multiplying external dimensions includes the wall material. Using full tank height instead of water depth also counts freeboard as water.

A 2 m × 1 m tank filled to 1 m holds 2,000 L in the rectangular model. If filled to 0.8 m instead, it holds 1,600 L. Curved walls, internal fittings and rounded corners require a more detailed capacity measurement.

Before using the estimate

This is a volume conversion, not a statement that a floor or stand can carry a tank. Water storage loads require a separate structural assessment.

Open the Water Tank Calculator and compare your measurements with the example.