Aquarium Volume Calculator
To find aquarium volume, multiply length × width × height in inches and divide by 231 for US gallons. A 24 × 12 × 16 inch tank holds about 20 gallons, or 75 litres, and the water weighs about 166 pounds.
How to measure your tank
Measure in a straight line with a tape, and use the same unit for every side. Inside measurements give the real water volume. If you can only measure the outside, the answer will be slightly high because it includes the thickness of the glass. For height, measure to where the water will actually sit, or leave the full height in and lower the fill level: most tanks are filled to about an inch below the rim.
Aquarium volume formulas
| Shape | Volume in cubic inches |
|---|---|
| Rectangle | length × width × height |
| Cube | side × side × side |
| Cylinder | 3.1416 × radius² × height |
| Half cylinder | half of the cylinder formula |
| Corner (quarter circle) | one quarter of 3.1416 × radius² × height, where the radius is the straight side |
| Hexagon | 2.598 × side² × height, for six equal sides |
| Bow front | (rectangle area + area of the curved front) × height |
Divide cubic inches by 231 to get US gallons. Multiply US gallons by 3.785 for litres, or by 0.833 for UK gallons. For the bow front, the calculator treats the curve as an arc of a circle between the two front corners, which matches most bow tanks closely.
Worked example
A rectangular tank 24 inches long, 12 inches wide and 16 inches high: 24 × 12 × 16 = 4,608 cubic inches. Divide by 231 and you get 19.9 US gallons, which is 75.5 litres. Filled to 90%, that is about 18 gallons. The water alone weighs about 166 pounds when full.
Common tank sizes
Typical outside dimensions for standard rectangular tanks, with the volume those measurements work out to. Makers' dimensions vary, so measure your own tank.
| Sold as (gallons) | Typical size (inches) | Calculated US gallons | Litres | Water weight (lb) |
|---|---|---|---|---|
| 10 | 20 × 10 × 12 | 10.4 | 39 | 87 |
| 20 | 24 × 12 × 16 | 19.9 | 76 | 166 |
| 29 | 30 × 12 × 18 | 28.1 | 106 | 234 |
| 40 | 36 × 15 × 16 | 37.4 | 142 | 312 |
| 55 | 48 × 13 × 20 | 54.0 | 205 | 451 |
| 75 | 48 × 18 × 21 | 78.5 | 297 | 655 |
What the number does not tell you
The result is the volume of the empty shape. In a set-up tank, gravel, rocks, driftwood and equipment take up space, so the water you add is usually less, often by a tenth or more in a heavily decorated tank. That matters when you dose water treatments or medication, which are measured against the water actually in the tank. Weight matters too. Water is heavy, and the tank, substrate and stand add to it, so check that the furniture and the floor can carry a large tank before you fill it. Salt water is slightly heavier than fresh water.
Aquarium Volume Calculator FAQ
How do I calculate the volume of my aquarium?
For a rectangular tank, multiply length × width × height in inches and divide by 231 to get US gallons. A 24 × 12 × 16 inch tank holds about 20 gallons.
How many gallons is my fish tank?
Measure the length, width and height, enter them above and pick the shape. The calculator gives US gallons, litres and UK gallons.
How much does a full aquarium weigh?
Fresh water weighs about 8.34 pounds per US gallon, so a 55 gallon tank holds roughly 460 pounds of water. The glass, gravel, rocks and stand add to that.
Should I measure the inside or the outside of the tank?
Inside measurements give the true water volume. Outside measurements include the glass and overstate the volume a little, usually by a few percent on a typical tank.
Why does my tank hold less than its labelled size?
Labelled sizes are nominal and often use outside dimensions. Gravel, rocks, equipment and the gap below the rim all reduce the water you actually add.
How many litres are in a gallon?
One US gallon is about 3.785 litres. One UK (imperial) gallon is about 4.546 litres, so UK gallon figures are smaller numbers for the same tank.
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Last updated: November 12, 2026 · How we check this calculator