Working out tank volume
Volume is simply length times width times height. In centimetres the result divided by 1,000 gives litres; in inches the cubic total divided by 231 gives US gallons. Measure the inside dimensions, and remember gravel, rocks and the water line below the rim mean the real water volume is a little less than the box figure.
How many fish can it hold?
A well-known rule of thumb for freshwater tanks is about one inch of adult fish per gallon (roughly one centimetre per litre). It's only a starting point — active or messy fish need more room, and a good filter and regular water changes matter more than hitting a number. Always stock for the adult size of the fish, not the size in the shop.
Cycle before you stock
A new tank needs a few weeks to build the bacteria that process fish waste. Add fish gradually and test the water, rather than filling a fresh tank at once. The volume figure here helps you choose a filter and heater rated for your tank.
Frequently asked questions
How many gallons is my tank?
Multiply length by width by height. In inches, divide by 231 for US gallons; in centimetres, divide by 1,000 for litres.
How many fish per gallon?
A rough guide is one inch of adult fish per gallon, but filtration, species and maintenance matter more. Stock lightly.
How do I calculate an aquarium's volume?
Multiply length × width × height in the same unit, then convert. In litres, use centimetres and divide by 1,000: a 60×30×30 cm tank holds 54 litres.
How many fish can my tank hold?
A rough guide is 1 cm of adult fish per litre of water, less for messy or active species. Filtration and the fish's needs matter more than the number.
Why is actual water volume less than the tank size?
Gravel, rocks, decorations and the gap below the rim all displace water, so a tank usually holds about 10–15% less than its nominal size.