How to calculate fish tank volume
Measure the inside of the tank if you can, since glass thickness takes a little off. Then volume is just the area of the base times the height. This fish tank volume calculator does the conversions for you, using exact unit definitions: one US gallon is 231 cubic inches, one liter is 1,000 cubic centimeters, and one UK gallon is 4.546 liters.
- Rectangle: length × width × height
- Cylinder: π × (diameter ÷ 2)² × height
- Corner tank (quarter circle): π × side² ÷ 4 × height
- Hexagon: 2.598 × edge² × height
- Bow front: a rectangle plus the curved bow, estimated as two thirds of length × the extra depth at the center
An example: a standard tank measuring 30 × 12 × 18 inches holds 6,480 cubic inches. Divided by 231, that’s about 28 US gallons, or 106 liters. It’s sold as a “29 gallon” because the outside dimensions are a touch bigger.
Common aquarium sizes
| Tank | Size (L × W × H) | US gallons | Liters | Water weight (lb) |
|---|---|---|---|---|
| 10 gallon | 20 × 10 × 12 in | 10.4 | 39 | 87 |
| 20 gallon long | 30 × 12 × 12 in | 18.7 | 71 | 156 |
| 29 gallon | 30 × 12 × 18 in | 28.1 | 106 | 234 |
| 40 gallon breeder | 36 × 18 × 16 in | 44.9 | 170 | 374 |
| 55 gallon | 48 × 13 × 21 in | 56.7 | 215 | 473 |
| 75 gallon | 48 × 18 × 21 in | 78.5 | 297 | 655 |
Sizes vary slightly between brands, so measure yours for an exact figure.
Real water volume is less than the tank size
Gravel, rocks, wood, decorations and the gap at the top all take up space. Once a tank is set up, the actual water is often 10 to 20% less than the full volume. The calculator shows that range. It matters when you’re dosing medicines or water treatments, where you should use the real water volume rather than the tank’s label size.
Don’t forget the weight
Water weighs about 8.34 lb per US gallon, or 1 kg per liter. A 55-gallon tank holds roughly 460 lb of water before you add gravel, rock and the tank itself. Make sure the stand and floor can take it, and place larger tanks along a wall or over a supporting beam where possible.
Common questions
How do I convert aquarium gallons to liters?
Multiply US gallons by 3.785. A 20 gallon tank holds about 76 liters. For UK gallons, multiply by 4.546.
Why does my “29 gallon” tank calculate as less?
Tanks are named by outside dimensions and rounded. The glass, the rim and the space at the top mean the usable volume is a little lower.
Should I measure inside or outside the glass?
Inside, if you can. Outside measurements slightly overestimate the volume.
How many fish can I keep?
It depends on the species, their adult size and your filtration, not just gallons. Research each species’ needs and add fish gradually.
Does this work for ponds?
For a pond with straight sides, yes. Irregular ponds are harder to measure, so averaging several depth readings gives a better estimate.
Sources
Last reviewed .