Volume grows with the cube of the side length, which is why a small change in size has an outsized effect. A 10-inch cube holds 4.3 gallons while a 12-inch cube holds 7.5 gallons, a 73% increase from only two more inches per side.
How to measure a cube tank
- Measure one inside edge. If the three edges differ by more than a few millimetres, use the rectangular calculator instead.
- Rimless cubes typically use thicker glass than framed tanks of the same size, so the interior can be noticeably smaller than the advertised size.
- Cube tanks are usually filled close to the rim; a half-inch gap still removes several percent of the volume on small cubes.
Always measure inside the glass. Exterior sizes include the panel thickness and any trim, which on larger tanks can remove 5 to 10 percent of the apparent volume. See the measuring guide for the full method.
Cube volume formula
V = S³
Cube the interior side length. A cube with 12-inch sides holds 1,728 cubic inches, which is exactly one cubic foot or 7.48 US gallons.
With inches, divide the cubic result by 231 to get US gallons and multiply gallons by 3.785 for litres. With centimetres, divide by 1,000 for litres directly.
Worked example
Inputs: Side Length 12 in
A 12-inch cube is 12 × 12 × 12 = 1,728 cubic inches, which is 7.48 US gallons or 28.3 litres.
Cube versus rectangular footprint
For a given floor area, a cube uses the shortest possible perimeter of glass, which keeps material costs down and gives the widest sight lines through the front panel. The trade-off is depth: a 20-inch cube is 20 inches front to back, which makes maintenance at the rear of the tank harder than in a rectangular tank of similar volume.
Getting to real water volume
The formula gives the geometric volume of the tank to the rim. Three things make the real water volume smaller, and the calculator above lets you account for two of them directly:
- Fill height. Most tanks run an inch or two below the rim. Enter the actual water line and the result scales to match.
- Substrate. Gravel and sand sit in the water column. The substrate option removes the solid fraction of the bed (62 percent of its depth for typical gravel) from the total.
- Rock, wood and equipment. Use the displacement calculator to subtract hardscape by weight and equipment by type.
Water weight
Fresh water weighs about 8.34 pounds per US gallon (1 kg per litre). Reef water at a specific gravity of 1.025 is about 8.55 pounds per gallon. The calculator shows both so you can check what your stand and floor will carry before adding glass, substrate and rock on top.