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How the calculation works
Measure inside the glass, from the bottom to the usual waterline. Length × width × water height gives the rectangular water envelope. Cubic centimeters divide by 1,000 to give liters; cubic inches divide by 231 to give US gallons. Nominal tank size often includes empty space above the waterline and the glass itself.
The optional displacement percentage subtracts an estimate for solid substrate, rock and equipment below the waterline. Water between grains still belongs in actual water volume. Do not subtract the entire substrate-bed volume. Add operating sump water separately and account for overflow plumbing when estimating a complete system.
Rounded output is a planning estimate, not a measurement of your filled aquarium. For an irregular tank, a reliable fill record or manufacturer geometry is better than treating it as a rectangle. Keep the same actual-volume basis when planning changes and scaling product labels.
Worked example
A 36 × 18 × 18 inch internal water envelope holds about 50.49 US gallons. With an illustrative 10% solid displacement estimate, actual water is about 45.44 US gallons, or 172.03 liters.
Common questions
Should I use the advertised tank size?
Use measured internal dimensions and water height when possible. Advertised size is useful for identification, but often overstates operating water volume.
Is displacement always 10%?
No. The example value is adjustable. Hardscape, substrate porosity and installed equipment vary; a fill record is more reliable than a generic percentage.