A tank’s volume in US gallons is its inside length x width x height in inches, divided by 231. It holds less water than that once the gravel goes in and the water stops below the top.
Tank shape
Measure inside the glass. Type a fraction as a decimal, such as 12.5.
Your tank holds
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Measuring your tank
Measure inside the glass, from one side to the other, front to back, and from the bottom glass to the top of the frame. For a round tank, measure straight across the middle for the diameter.
Outside sizes, such as the ones in the fish tank sizes chart, include the glass and the frame. Aqueon’s 20 Long is 30¼ x 12½ x 12¾ in outside, which works out to 20.9 gallons for the whole box, more than the water inside it.
Gallons, liters and UK gallons
A US gallon is 231 cubic inches, or 3.785 liters, and a liter is 1,000 cubic centimeters. A UK gallon is 4.546 liters, about 20% bigger than a US gallon.
Two tanks with the same gallons can have very different floors. See why gallons don’t tell you the floor size, and check a tank against your pet’s minimum with the enclosure size checker.
How heavy a full tank is
A US gallon of fresh water weighs about 8⅓ lb, and a liter about 1 kg, so a 55 gallon tank holds about 460 lb of water before you add the glass, gravel and stand. Put the tank on an aquarium stand sized and designed for it, one that meets all weight and safety specifications.
Sizing a heater
Heater makers rate their heaters by tank size, in gallons or liters. Size one for your tank with the aquarium heater calculator.