How to use this calculator
Measure inside the glass and use actual water height, not tank height. Enter an estimated displaced water volume for gravel, rocks, and equipment. Water added during filling can give a more accurate real-world measurement.
The formula
A worked example
A tank with internal measurements 60 × 30 cm and 35 cm of water holds 63 L before displacement. Subtracting 5 L leaves 58 L, about 15.32 U.S. gallons.
Assumptions & limitations
For rectangular tanks only. Displacement is user-estimated and may be uncertain. The result is a volume estimate, not advice on stocking, filtration, or medication dosing.
Common questions
Why is this lower than the tank’s advertised capacity?
Advertised volume may use outside dimensions and full height, without subtracting glass, headspace, or decor.
Are these Imperial gallons?
No. The calculator explicitly uses U.S. liquid gallons, each equal to 3.785411784 liters.
Units reference
The calculation follows the method above. This reference supports the units and conversions.
NIST: units and conversion factors ↗Formula and worked example checked against automated test cases. About our methodology.