What is Fish Tank Volume?
Fish tank volume is the total amount of water an aquarium can hold, measured in litres, gallons or cubic units. It is calculated from the tank's internal dimensions and is the single most important number in fishkeeping, because dosing, heater choice, filtration and stocking all depend on it.
Tank volume vs actual water volume
There is a difference between tank volume and actual water volume. Tank volume is the full internal capacity, but the real water volume is usually 10 to 15 percent lower once substrate, rocks, decorations and the gap at the top are accounted for. For a 60 by 30 by 30 cm tank the full volume is 54 litres, while the real water volume is closer to 46 to 49 litres.
Why fish tank volume matters
Knowing your exact volume matters for several reasons. Medications and water conditioners are dosed per litre or gallon, so the wrong volume means the wrong dose. Heater wattage, filter turnover and the number of fish you can safely keep all scale with volume. Even choosing a stand depends on the filled weight, which comes from the volume.
How fish tank volume is measured
To find your volume, measure the inside length, width and height of the tank and multiply them together, then convert to litres or gallons. Use the calculator at the top of the site for any shape, or the formulas on our fish tank volume formula page to work it out by hand.