How many gallons is my fish tank?
US gallons = cubic inches / 231. One US gallon is 231 cubic inches by definition, so this part is exact for whatever box you measured. The uncertain part is the measuring. Use the inside of the water: length, width, and the height from the bottom of the water (or the top of the gravel) to the water surface.
Liters = US gallons x 3.785411784. Imperial gallons = liters / 4.54609. A result in the low teens of US gallons is a smaller number of imperial gallons. The word "gallon" on this page means the US gallon.
How many gallons is a 30x12x12 tank?
30 x 12 x 12 = 4,320 cubic inches. 4,320 / 231 = 18.70 US gallons. Open that size in the calculator with the preset, or load 30 x 12 x 12.
How many gallons is a 36x18x18 tank?
36 x 18 x 18 = 11,664 cubic inches. 11,664 / 231 = 50.49 US gallons. Load 36 x 18 x 18. A water box of 20 x 10 x 12, the size people quote for a 10 gallon, is 10.39 US gallons. The tank on the shelf is a bit different. See the 10 gallon fish tank dimensions.
Other shapes
- Cylinder: pi x radius x radius x height. Diameter 12 inches and water height 18 inches is 8.81 US gallons.
- Bow-front: the rectangular part (length x end depth) plus the circular segment of the bow. The radius of that segment is bulge/2 + length squared / (8 x bulge), where bulge is center depth minus end depth. A 36 inch tank, end depth 12, center depth 16, water height 16, is 36.64 US gallons.
- Hexagon: flat-to-flat width F has area (square root of 3) / 2 x F squared. Point-to-point and side length use the matching regular-hexagon formulas, and they agree when the measurements are of the same tank.
- Corner, quarter circle: pi x side x side x height / 4.
- Corner, right triangle: side A x side B x height / 2. This is only for a 90 degree back corner.
Water weight
Fresh water is 8.34 lb per US gallon here. Salt water multiplies by the specific gravity (1.025 unless you type another). Aqueon's setup guide uses about 10 lb per gallon as a planning weight for a filled aquarium with gravel and decorations. This page shows that 10 lb figure from the full water volume, before you subtract rock. Rock is heavier than the water it displaces, so the stand load goes up, not down, when you add rock.
The stand has to be an aquarium stand, level, and rated for that weight. A desk or a dresser is a common way tanks fail.
Frequently asked questions
How many gallons is my fish tank?
Measure the water, then pick the shape. A rectangle is length times width times water height, in inches, divided by 231. That quotient is US gallons. Multiply by 3.785 to get liters.
How many gallons is a 30x12x12 tank?
If those inches are the water, 30 x 12 x 12 / 231 = 18.70 US gallons. A tank sold as a 20 gallon long is larger on the outside than 30 x 12 x 12, so do not treat this as that model's rated capacity.
How many gallons is a 36x18x18 tank?
36 x 18 x 18 / 231 = 50.49 US gallons when those are internal water dimensions. An older catalog rounded a 50 gallon tank to about 36 x 18 x 18. The current outside chart on the dimensions page is a different measurement.
How much does aquarium water weigh?
Fresh water is taken as 8.34 lb per US gallon. Salt water at specific gravity 1.025 weighs 8.34 x 1.025 lb per gallon. A planning figure for the whole setup, including glass, gravel, and rock, is about 10 lb per gallon of water. Rock that displaces water adds weight on top of that.