Fish Tank Volume Calculator in Gallons

Calculating fish tank volume in gallons

To find gallons, calculate the volume in cubic inches (length times width times height in inches) and divide by 231 for US gallons or by 277.4 for UK gallons. This calculator shows both at once so you never mix them up.

How to use this gallon calculator

1. Pick the shape. Most tanks are rectangular. Choose bow front, corner, cylinder or hexagon if yours is curved or angled.
2. Choose inches as the unit if you measured with an imperial tape. You can also enter centimetres or feet; the result is still shown in gallons.
3. Enter the inside length, width and height. Measure the height to where the water actually sits if you want the real amount of water rather than the full capacity.
4. Read US gallons or switch to UK gallons in the result box. Both are calculated from the same measurements.

Worked example: how many gallons is a 36 x 18 x 18 tank?

A tank that measures 36 inches long, 18 inches front to back and 18 inches tall works out like this:

• 36 x 18 x 18 = 11,664 cubic inches
• 11,664 / 231 = 50.5 US gallons
• 11,664 / 277.4 = 42.0 UK gallons (about 191 litres)

Those are outside measurements. With 1/4 inch glass the inside is about 35.5 x 17.5 x 17.75 inches, which is 47.7 US gallons. Fill it to one inch below the rim and you have about 45 US gallons of water. So a tank that “looks like” 50 gallons holds closer to 45 once it is running.

Gallons from feet instead of inches

If you measured in feet, multiply the three sides to get cubic feet, then multiply by 7.48 for US gallons or 6.23 for UK gallons. The same 36 x 18 x 18 inch tank is 3 x 1.5 x 1.5 feet, or 6.75 cubic feet, and 6.75 x 7.48 = 50.5 US gallons, the same answer as above. The fish tank volume calculator in feet does this step for you.

Which gallon figure should you use?

• Buying a heater, filter or light: use the full tank size in gallons. Product boxes are labelled that way.
• Dosing medication, salt or water conditioner: use the real water volume, which is lower. Read the dosing label to see whether it means US or UK gallons. A UK gallon is about 20 percent larger than a US gallon, so mixing them up changes the dose by roughly a fifth.
• Checking a stand or floor: use the full volume. Water weighs about 8.3 lb per US gallon.

Check the result with a bucket

If you want to confirm the number for your own tank, fill it with a container of known size, such as a 1 gallon jug or a 2 gallon bucket with a marked line, and count how many times you pour. This takes longer, but it is the only way to get the exact figure for a tank that is already furnished with gravel and rock.

Common gallon mistakes

• Dividing centimetres by 231. The 231 rule only works for cubic inches. For centimetres, divide by 1,000 for litres, then by 3.785 for US gallons.
• Trusting the name on the box. A “29 gallon” tank is about 28.1 gallons from its outside size, and a “40 breeder” is closer to 45. The name is a rounded label.
• Measuring over the plastic rim. The black trim adds height but holds no water.

For a step-by-step guide with more examples, read how many gallons is my fish tank. To identify a tank from its footprint, see standard fish tank dimensions.