Use internal water dimensions, not just the outside of the glass or the model name.
The most useful rectangular-tank calculation uses internal length, internal width and the actual water height. External measurements include glass, trim and framing. Full glass height can include an air gap that never contains water.
Measure in centimetres, multiply length by width by water height and divide by 1,000. For example, 90 × 35 × 40 cm equals 126 litres before displacement.
Original diagram supporting the how to calculate aquarium volume properly explanation.
One UK imperial gallon is 4.54609 litres. One US liquid gallon is 3.785411784 litres. A 126-litre tank is about 27.7 UK gallons but 33.3 US gallons, so simply saying “gallons” can create a large error.
Keep the tape square, measure to the real water line and decide whether you need geometric water space or an estimate after displacement. For cylinders and bow-front tanks, use the appropriate shape formula or manufacturer data rather than forcing rectangular maths.
Volume is useful for percentage water changes, filter-flow comparison, water-mass estimates and product instructions. It does not replace species-specific care or compatibility research.
If a result looks surprising, repeat those three checks before changing the formula. A calculator cannot correct a measurement taken from the wrong physical points.