The radius is half the width of the flat back. Enter it with the inside height and the calculator returns exactly half the volume of the equivalent full cylinder.
How to measure a half cylinder tank
- Measure the inside length of the flat back panel and divide by two for the radius.
- Check that the front-to-back depth at the centre matches the radius; if it is noticeably less, the tank is a bow front and that calculator fits better.
- Measure height on the flat back where the glass is straight.
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.
Half Cylinder volume formula
V = π × r² × H ÷ 2
Half the volume of a full cylinder with the same radius and height. The radius is half the length of the flat back panel.
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: Radius 12 inHeight 20 in
π × 12² × 20 ÷ 2 = 4,524 in³, which is 19.6 US gallons or 74.1 litres.
Half cylinder versus full cylinder
Halving the shape halves the water but keeps the full curved viewing panel, so a half cylinder is one of the most volume-efficient ways to get a panoramic front. It also keeps the centre of mass close to the wall, which matters for wall-mounted and stand-mounted installations.
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.