Enter the two wall-side lengths. They do not need to be equal, although most commercial corner tanks are symmetric.
How to measure a corner prism tank
- Measure each wall side from the inside corner to the point where the front panel begins.
- Do not measure the front panel; it is the hypotenuse and is not used in the formula.
- If the front panel is curved, use the corner bow front or corner cylinder calculator instead.
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.
Corner Prism volume formula
V = ½ × a × b × H
A right triangle has half the area of the rectangle formed by its two legs. Multiply that area by the height.
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: Leg 1 (wall side) 24 inLeg 2 (wall side) 24 inHeight 20 in
½ × 24 × 24 × 20 = 5,760 in³, which is 24.9 US gallons or 94.4 litres.
A corner prism is half a rectangle
Cutting a square 24 × 24 tank along its diagonal gives two corner prisms. That makes it easy to compare with box tanks: a corner prism with 24-inch legs holds exactly half of a 24 × 24 tank of the same height. It also means the substrate footprint is half the leg product, which the substrate option accounts for automatically.
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.