Because a sphere narrows toward the top and bottom, the water volume is not proportional to depth. Filling a bowl to three quarters of its height gives much more than three quarters of its full volume.
How to measure a fish bowl tank
- Measure the widest inside diameter, usually about a third of the way up from the base.
- If the bowl stands on a flat circle of glass, measure that circle for the flat base diameter.
- Measure the water height from the inside floor to the surface; the calculator uses the spherical segment formula for the partial fill.
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.
Fish Bowl volume formula
V = π × d³ ÷ 6
The volume of a sphere. For a bowl with a flat base, the calculator removes the spherical cap below the base. Water height is measured from the inside floor and uses the spherical segment formula.
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: Diameter 10 in
A 10-inch bowl is π × 10³ ÷ 6 = 524 in³ as a full sphere, which is 2.27 US gallons or 8.6 litres. Filled to the usual level below the opening it holds closer to 1.9 gallons.
Why the sphere formula overstates usable volume
A complete sphere has no opening and no flat base. A real bowl loses the cap at the top where the opening is cut and the cap at the bottom where it is flattened, and nobody fills water to the very brim. Expect the usable volume to be 70 to 85 percent of the full sphere figure, which is why the fill height and flat base options exist.
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.