tank
Aquarium volume calculator
Work out how much water a tank actually holds, from its dimensions — including the substrate and freeboard that the label ignores.
47.9 L of actual water — 12.6 US gal, 10.5 Imp gal
The tank’s gross volume is 64.8 L. Dosing to the label rather than to the actual water volume over-doses by 26%.
Working — every step, so you can check it
- Gross volume
- 60 × 30 × 36 ÷ 1000 = 64.8 L
- Water height
- 36 − 5 substrate − 3 freeboard = 28 cm
- Net water volume
- 60 × 30 × 28 ÷ 1000 = 50.4 L
- Displacement allowance
- 50.4 × 5% = 2.5 L
- Actual water volume
- 50.4 − 2.5 = 47.9 L
- In gallons
- 12.6 US gal • 10.5 Imp gal
Formula
- Gross volume (L) = length (cm) × width (cm) × height (cm) ÷ 1000
- Water height (cm) = height − substrate depth − freeboard
- Net water volume (L) = length × width × water height ÷ 1000
- Displacement allowance (L) = net water volume × displacement fraction
- Actual water volume (L) = net water volume − displacement allowance
- US gallons = litres ÷ 3.785411784 • Imperial gallons = litres ÷ 4.54609
Assumptions
An estimate whose assumptions are invisible is indistinguishable from a guess.
- The tank is a rectangular box with flat panels. Bow-front and curved tanks hold slightly more than this calculation suggests.
- Dimensions are internal. Glass thickness reduces internal dimensions by roughly twice the pane thickness on each axis.
- Substrate is level at the stated depth.
Limitations
Where this calculation stops being reliable. Read these before acting on a result.
- Manufacturers quote gross volume, so a '60 litre' tank commonly holds 45–50 litres of actual water once substrate, hardscape and freeboard are accounted for. Dosing to the label over-doses.
- Displacement is an estimate, not a measurement. If precision matters — for medication dosing especially — measure by filling with a known container.
- The calculation does not apply to irregular, cylindrical or corner tanks.