Standard aquarium sizes
Glass aquariums are made in a small set of standard sizes. In North America they are named by nominal US gallons; in Europe, the UK and Australia they are sold by their dimensions in centimetres or by litres. The calculator searches both lists and shows the six closest to the volume you asked for.
The dimensions below are the usual outside measurements. Makers differ by a few millimetres, and the plastic trim on framed tanks adds about half an inch, so measure your own stand or shelf before buying.
| US tank | L × W × H (in) | L × W × H (cm) | Litres |
|---|---|---|---|
| 5.5 gallon | 16 × 8 × 10 | 41 × 20 × 25 | 21 |
| 10 gallon | 20 × 10 × 12 | 51 × 25 × 30 | 38 |
| 20 gallon high | 24 × 12 × 16 | 61 × 30 × 41 | 76 |
| 20 gallon long | 30 × 12 × 12 | 76 × 30 × 30 | 76 |
| 29 gallon | 30 × 12 × 18 | 76 × 30 × 46 | 110 |
| 40 gallon breeder | 36 × 18 × 16 | 91 × 46 × 41 | 151 |
| 55 gallon | 48 × 13 × 21 | 122 × 33 × 53 | 208 |
| 75 gallon | 48 × 18 × 21 | 122 × 46 × 53 | 284 |
| 125 gallon | 72 × 18 × 21 | 183 × 46 × 53 | 473 |
See the full list in the standard aquarium sizes chart.
Working out a custom tank
For a rectangular tank, any one dimension follows from the volume and the other two:
Worked example. You have a shelf 120 cm long and 40 cm deep and want 200 litres. 200,000 ÷ (120 × 40) = 41.7 cm of height, brim-full. Add a few centimetres so the water line can sit below the rim.
Shape matters as much as volume
Two tanks of the same volume are not equal for fish. A 20-gallon long and a 20-gallon high hold the same water, but the long has 25% more floor and water surface. More surface means more oxygen exchange, and more length means more room to swim. For most fish, choose the longest, widest footprint you have space for, and treat height as the least important dimension. Tall, narrow tanks suit only a few species, such as angelfish.
"Breeder" tanks (30 and 40 gallon breeder) are the low, wide versions and are favourites for exactly this reason.
Common mistakes
- Choosing by gallons alone. Check the footprint: active fish need length, and bottom dwellers need floor area.
- Forgetting the stand. The stand must support the whole bottom frame and is usually a little bigger than the tank.
- Not allowing space around the tank. Leave about 10 cm (4 in) behind for filters and cables, and room above to lift the lid and reach in.
- Assuming every brand is identical. Check the maker's own measurements before ordering a lid, light or stand.
Questions and answers
What are the dimensions of a 55 gallon tank?
A standard 55-gallon aquarium is 48 inches long, 13 inches wide and 21 inches high (about 122 × 33 × 53 cm). It holds roughly 208 litres nominally.
What are the dimensions of a 20 gallon tank?
There are two: the 20 gallon high is 24 × 12 × 16 inches (61 × 30 × 41 cm) and the 20 gallon long is 30 × 12 × 12 inches (76 × 30 × 30 cm). The long version is better for most fish.
What size is a 10 gallon tank?
A standard 10-gallon aquarium is 20 × 10 × 12 inches (51 × 25 × 30 cm) and holds about 38 litres.
How do I find tank dimensions from gallons?
Multiply the gallons by 231 to get cubic inches, then divide by the length and width you have room for. The result is the height needed. Or enter the volume above to see the standard sizes that match.
Is a long tank better than a tall tank?
For most fish, yes. A longer tank has more water surface for oxygen and more swimming room. Tall tanks hold fewer fish for the same volume.