Volume is length times width times height — then a unit divide
A rectangular tank's volume is just its three inside dimensions multiplied together. There are 231 cubic inches in a US gallon and 1,000 cubic centimetres in a litre, so one division gives you the answer. For a cylinder, use π × radius² × height instead.
Standard tank sizes
| Name | Dimensions (in) | Litres | Water weight |
|---|---|---|---|
| 10 gal | 20 × 10 × 12 | 37.9 L | 83 lb |
| 20 gal high | 24 × 12 × 16 | 75.7 L | 167 lb |
| 29 gal | 30 × 12 × 18 | 109.7 L | 242 lb |
| 40 gal breeder | 36 × 18 × 16 | 151.4 L | 334 lb |
| 55 gal | 48 × 13 × 21 | 208.2 L | 459 lb |
| 75 gal | 48 × 18 × 21 | 283.9 L | 626 lb |
| 125 gal | 72 × 18 × 22 | 473.2 L | 1,043 lb |
Water weight uses 8.34 lb per US gallon. Add glass and substrate, then check the stand: a filled 55-gallon tank pushes past 600 lb on its own.
Why the real number is lower than the box says
- Measure inside. Use the internal dimensions. Glass walls remove a few percent, and decor, filter and heater remove more.
- You don't fill to the rim. Tanks run at about 90% fill, so actual water is roughly a tenth below the geometric volume.
- US or UK gallon. A US gallon is 3.785 L; an imperial gallon is 4.546 L. The 20% gap ruins medication doses and stocking estimates if confused.
- Substrate displaces water. A 5 cm gravel bed cuts water volume by about 5 to 7%; live rock in a reef tank can take 20 to 30%.
Common questions
How do I calculate the volume of a rectangular tank?
Multiply length by width by height. In inches, divide the result by 231 for US gallons; in centimetres, divide by 1,000 for litres.
How much does the water in my aquarium weigh?
Fresh water weighs about 8.34 pounds per US gallon, or 1 kg per litre. A 60-litre tank holds about 60 kg (132 lb) of water alone, before glass and substrate.
Should I measure the inside or the outside of the tank?
Always the inside. Glass thickness can shave a few percent off the true volume, and substrate, rocks and equipment take up more, so real water volume is often 10 to 20% below the raw figure.
What is the difference between US and UK gallons?
A US gallon is 3.785 litres and an imperial (UK) gallon is 4.546 litres, about 20% larger. Mixing them up throws dosing and stocking well off.
