The calculator splits the tank into a box and a half cylinder. If the bullnose radius equals half the width, the curve meets the sides smoothly; if it is smaller, there is a short straight section on each side of the nose.
How to measure a bullnose tank
- Overall length is from the outermost point of the curve to the inside of the flat end panel.
- Radius is the distance from the centre of the curve to its edge. For a smooth bullnose it equals half the inside width.
- Overall length must be at least the radius; otherwise the curved section cannot fit.
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.
Bullnose volume formula
V = ((L − r) × W × H) + (π × r² × H ÷ 2)
The rectangular section runs from the flat end to the start of the curve (length minus radius). The rounded end is a half cylinder with the given radius. 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: Overall Length 48 inWidth 18 inBullnose Radius 9 inHeight 20 in
Box: (48 − 9) × 18 × 20 = 14,040 in³. Nose: π × 9² × 20 ÷ 2 = 2,545 in³. Total 16,585 in³ = 71.8 US gallons (271.8 litres).
Bullnose versus round end
A round end (stadium) tank has semicircles at both ends; a bullnose has one. For the same overall length and width, the bullnose holds slightly more water because one end stays square. The difference is the volume of the corners the second semicircle would trim away: (1 − π ÷ 4) × r² × H, about 21% of a square section the size of the radius.
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.