How the volume is worked out
Volume is length multiplied by width multiplied by depth, which gives cubic metres. One cubic metre is 1,000 litres.
The litres are then divided by the size of a gallon to give both figures. An imperial gallon is 4.546 litres and a US gallon is 3.785, so a tank sold as 48 gallons in the US is about 40 imperial gallons.
Use inside dimensions. A tank measured on the outside loses a centimetre or two on each side to the glass, which matters on a small tank.
Worked example
A tank 90 cm by 45 cm with 45 cm of water holds 0.18 cubic metres, which is 182 litres, 40.1 imperial gallons or 48.1 US gallons.
A curved front, a bow front or a corner tank holds less than its bounding box, so treat a rectangular calculation as an upper limit.
Why the gallon figure matters
Most guidance for Choosing a filter, stocking fish or dosing a treatment is written in gallons, and which gallon is rarely stated. A 20 per cent error in volume becomes a 20 per cent error in dose.
Litres are unambiguous, so it is worth converting any gallon figure to litres before acting on it.
Displacement is the other correction. Gravel, rock and equipment take up volume, so the water a tank actually holds is less than the calculation.
Limits of this calculator
It assumes a rectangular box. A bow front, a cylinder, a corner tank or a sloping side needs its own geometry.
It does not subtract the volume taken by substrate, rock, filters or equipment, which can be 10 to 20 per cent of a planted tank.
It does not account for the freeboard below the rim or for the water level you actually run at.
For the liner a pond of this size needs, use the pond liner calculator.
Inputs for the volume of aquarium calculator
Tank length
Inside length of the tank in metres, 0.9 m for a 90 cm tank.
Tank width
Inside width of the tank in metres, 0.45 m for a 45 cm tank.
Water depth
Depth of water in metres, not the height of the glass. A tank is rarely filled to the rim.
CALCULATOR QUESTIONS
Volume of aquarium calculator questions
How many litres are in a gallon?
4.546 in imperial measure and 3.785 in US measure, which is why both are shown.
Should I measure inside or outside?
Inside. The glass thickness makes a real difference on a small tank.
Does gravel reduce the volume?
Yes. Substrate and rock displace water, so a tank holds less than its dimensions suggest.
How do I measure an irregular pond?
Take several depths and average them, and treat the result as an estimate rather than a precise figure.