Tank Volume Calculator: How Many Gallons Is My Aquarium?

Bought a second-hand tank with no label? Building one? Enter the outside measurements and get the volume in US gallons and litres — the number every other calculation (stocking, heating, dosing, water changes) starts from.

The formula

For a rectangular tank: length × width × height (in inches) ÷ 231 = US gallons. (There are 231 cubic inches in a US gallon.) Using centimetres instead: length × width × height ÷ 1,000 = litres.

Two honest adjustments: outside measurements include glass thickness, and nobody fills a tank to the brim — substrate, décor and a sensible waterline typically displace 10–15% of the nominal volume. For dosing medication or treatments, work with about 90% of the figure this calculator gives. For choosing fish, what matters just as much is below.

Gallons aren't the whole story: footprint matters

Two 20-gallon tanks can have completely different shapes — the standard "high" is 24″ long and 16″ tall; the "long" is 30″ long and 12″ tall. Same gallons, different lives: active schoolers and bottom-dwellers use length and floor area, so the long tank suits them better, while tall fish like angelfish use height. Before you stock by volume alone, check your exact fish list in the stocking calculator — it also enforces each species' minimum tank size — and read why simple rules mislead.

Standard tank sizes (nominal)

Dimensions are the common standard sizes; individual manufacturers vary slightly — measure your own tank for an exact answer.