Aquarium volume calculator

Every shape comes down to the area of its footprint multiplied by its height. Only the footprint changes:

• Rectangle or cube: length × depth.
• Cylinder: π × radius², where the radius is half the diameter.
• Bow-front: the rectangle behind the curve (length × depth at the ends), plus the segment of a circle the bow adds, worked out from the length and how far the centre bulges beyond the ends.
• Corner (quarter circle): π × side² ÷ 4, where the side is the length of one straight back pane.
• Hexagon: 0.866 × width², with the width measured across two flat faces rather than from point to point.

Multiply the footprint in square centimetres by the height in centimetres and divide by 1,000 for litres. Measurements in inches are converted at 2.54 cm to the inch first.

The volume printed on the box normally describes the outside dimensions filled to the brim, and nobody runs a tank like that. The glass takes up space on every wall, anything from 4 mm on a nano to 15 mm or more on a large tank. The water line usually sits a few centimetres below the rim, and the substrate fills the bottom of the tank.

Those three add up quickly. A 60 × 30 × 36 cm tank is 64.8 litres on paper. With 6 mm glass, the water 3 cm below the rim and 5 cm of substrate, it holds about 46 litres, which is close to 30% less.

Rock, wood and equipment take up more room still. They are not counted here because they are too irregular to measure; if you need an exact figure, fill the tank from a container of known size and keep count.

Medication, fertiliser and water conditioner are dosed per litre or per gallon, so dosing on the nominal volume overdoses by the difference. Heaters and filters are sized by volume too, and so is how many fish the tank can hold. The button under the result carries your water volume straight into the stocking calculator.

Weight matters as well. Fresh water weighs a kilogram per litre, so even a 200-litre tank puts more than 200 kg on its stand before the glass, rock and substrate are added. Check what your stand and floor can carry before filling a large tank.

A US gallon is 3.785 litres and a UK (imperial) gallon is 4.546 litres, so the same tank measures about 20% more in US gallons than in UK gallons. Most tank sizes quoted online and on American packaging are US gallons; older British tanks are often sold in imperial gallons. The result shows both, so you never have to guess which one a label meant.