How to use the aquarium volume calculator
- Choose the tank shape.
- Measure the inside of the tank. The glass thickness adds up on big tanks.
- Enter the gap between the water and the rim, and the depth of your gravel or sand.
- Read the full volume, the real water volume and its weight. Switch to metric to measure in centimetres and see litres.
The formula
Volume = base area × height. The base area depends on the shape:
- Rectangle: length × width
- Cylinder: π × (diameter ÷ 2)²
- Hexagon (regular): 2.598 × side²
- Corner (quarter circle): π × side² ÷ 4
Then convert: US gallons = cubic inches ÷ 231. One US gallon is defined as exactly 231 cubic inches (NIST). In metric, cubic centimetres ÷ 1,000 = litres. A UK gallon is 4.546 litres, so it is about 20% bigger than a US gallon.
Real water uses the height minus the gap at the top and the gravel layer. Rocks, wood and decorations take up more space, so the true water volume is a little lower again.
Water weight = litres × 0.998 kg (fresh water at about 20 °C).
Tips
- Dose by real water. Water conditioners and treatments should be dosed for the water you actually have, not the label size.
- Heaters and filters are sized from the volume. See the aquarium heater calculator.
- Floors and stands. A large tank can weigh several hundred pounds when full. Put it on a stand made for it.
Sources
- NIST Handbook 44, Appendix C: 1 US gallon = 231 cubic inches; 1 UK (imperial) gallon = 4.54609 litres.
- Density of water at 20 °C: 0.998 g/cm³.
Frequently asked questions
How many gallons is my fish tank?
Multiply the inside length × width × height in inches and divide by 231, because one US gallon is exactly 231 cubic inches (NIST). For litres, measure in centimetres and divide by 1,000.
How do I calculate the volume of a fish tank?
For a rectangle, length × width × height. For a cylinder, 3.1416 × radius² × height. For a hexagon, 2.598 × side² × height. For a corner (quarter-circle) tank, 0.785 × side² × height. Then convert: ÷ 231 for US gallons from inches, or ÷ 1,000 for litres from centimetres.
How many litres is my fish tank?
Measure in centimetres and multiply length × width × height, then divide by 1,000. A 60 × 30 × 40 cm tank holds 72,000 cm³ = 72 litres. One US gallon is 3.785 litres and one UK gallon is 4.546 litres.
How much does a full fish tank weigh?
Fresh water weighs about 8.3 lb per US gallon (about 1 kg per litre), so multiply the water volume by 8.3 for pounds. Then add the glass, gravel, rocks and stand, and make sure the furniture under it can take the load.
How many fish can I put in a 10 gallon tank?
There is no exact number: it depends on the fish’s adult size, how active they are and your filter. The old “one inch of fish per gallon” rule is only a rough start and fails for big-bodied or messy fish. Check each species’ minimum tank size before buying.