How Many Gallons Is My Fish Tank? Common Tank Sizes Chart
To find how many US gallons a rectangular fish tank holds, measure it in inches and use:
US gallons = length × width × height ÷ 231
One US gallon is exactly 231 cubic inches (NIST). Measure the inside of the glass if you can, multiply the three numbers, and divide by 231. The aquarium volume calculator does this for rectangles, cylinders, hexagons and corner tanks, in gallons and litres.
Common fish tank sizes chart
Tanks are sold by their nominal size in US gallons, but the outside dimensions for the same size differ between brands, so measure your own tank rather than trusting a size chart. This chart converts the common nominal sizes into litres and the weight of the water alone, using 1 US gallon = 3.785 litres (NIST) and fresh water at 0.998 kg per litre:
| Tank size | Litres | Weight of the water when full |
|---|---|---|
| 5 gallon | 19 L | 42 lb (19 kg) |
| 10 gallon | 38 L | 83 lb (38 kg) |
| 20 gallon | 76 L | 167 lb (76 kg) |
| 29 gallon | 110 L | 242 lb (110 kg) |
| 40 gallon | 151 L | 333 lb (151 kg) |
| 55 gallon | 208 L | 458 lb (208 kg) |
| 75 gallon | 284 L | 625 lb (283 kg) |
| 90 gallon | 341 L | 750 lb (340 kg) |
| 125 gallon | 473 L | 1,041 lb (472 kg) |
The glass, gravel, rocks and stand add a lot more on top, and a tank is never filled to the brim, so the real water volume is lower than the nominal size.
How many litres is my fish tank?
Measure in centimetres and multiply length × width × height, then divide by 1,000 (1 litre = 1,000 cm³). To convert: one US gallon is 3.785 litres, and one UK (imperial) gallon is 4.546 litres, so a UK gallon is about 20% bigger than a US gallon.
How to get the volume of other tank shapes
Find the area of the base, multiply by the height, then convert to gallons or litres:
- Rectangle: length × width
- Cylinder: 3.1416 × (diameter ÷ 2)²
- Hexagon (regular): 2.598 × side²
- Corner (quarter circle): 3.1416 × side² ÷ 4
The real water in your tank
The number on the box is the size of the glass, not the water. To find the water you actually have, use the height of the water rather than the tank, and subtract the gravel layer. Rocks, wood and decorations take up space too.
Real water (US gallons) = length × width × (water depth − gravel depth) ÷ 231, all in inches.
That matters when you dose water conditioner or medicine, which should be measured for the water you actually have.
How much does a full tank weigh?
Fresh water weighs about 8.3 lb per US gallon (about 1 kg per litre), so multiply your real water volume by 8.3 for pounds, before adding the glass, gravel, rocks and stand. Put any tank on a stand made for it.
How many fish can I put in my tank?
There is no exact number. It depends on the fish’s adult size, how active they are and how good your filter is. 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.
Next steps
- Heater size: heaters are sized from the volume. See the aquarium heater calculator.
- Exact volume: measure your tank and use the aquarium volume calculator.
Sources: NIST Handbook 44, Appendix C (1 US gallon = 231 cubic inches; 1 UK gallon = 4.54609 litres); density of fresh water at 20 °C, 0.998 g/cm³. Litres and water weights in the chart are unit conversions of the nominal size.