Enter the inside radius and height. If the tank is a tall column with a decorative base or cap, measure only the water-holding section.
How to measure a cylinder tank
- Measure the inside diameter across the widest point and divide by two for the radius.
- Acrylic cylinders often have thick walls; subtract both wall thicknesses from the outside diameter if you cannot measure inside.
- Height is the interior water-holding height, excluding any pedestal or lighting hood.
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.
Cylinder volume formula
V = π × r² × H
Multiply the circular floor area (π times radius squared) by the height. If you measured the diameter, halve it before entering.
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 10 inHeight 24 in
π × 10² × 24 = 7,540 in³, which is 32.6 US gallons or 123.6 litres.
Why radius errors matter more than height errors
Because the radius is squared, a 5% error in radius produces roughly a 10% error in volume, while a 5% error in height produces only a 5% error. Measure the diameter in two directions and average them, and be careful to measure inside the acrylic rather than around the outside.
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.