This calculator models the tank as a box plus a half-ellipse. Enter the flat depth from the back wall to the point where the curve starts, then the extra bow depth at the centre of the front panel. The result includes both sections.
How to measure a bow front tank
- Flat depth: measure from the inside of the back glass to the inside of the front glass at the extreme left or right corner, where the curve begins.
- Bow depth: measure the total depth at the centre of the front panel and subtract the flat depth.
- Length is the straight inside measurement across the back wall.
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.
Bow Front volume formula
V = (L × D × H) + (π × (L/2) × B × H ÷ 2)
The tank is split into a rectangular box (length × flat depth) plus a half-ellipse whose semi-axes are half the length and the bow depth. Both are multiplied by height and added.
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: Length 36 inFlat Depth 12 inBow Depth 4 inHeight 20 in
Box: 36 × 12 × 20 = 8,640 in³. Bow: π × 18 × 4 × 20 ÷ 2 = 2,262 in³. Total 10,902 in³ = 47.2 US gallons (178.6 litres). The bow adds about 9.8 gallons.
How much the bow adds
The curved section is worth π ÷ 4, roughly 78.5%, of a rectangular strip with the same length and bow depth. On a typical 36-inch tank with a 4-inch bow that is close to 10 gallons, enough to change heater wattage and medication dosing. The deeper the bow relative to the flat depth, the larger the share it represents.
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.