Each shape has its own volume formula
Pick the shape, plug in the dimensions in one consistent unit, and the volume follows. Radius is half the diameter, so if a tank is quoted as "1 m" that is usually diameter, not radius.
| Shape | Volume |
|---|---|
| Vertical / horizontal cylinder | V = π × r² × h |
| Rectangular | V = width × depth × height |
| Oval (elliptical) | V = (π ÷ 4) × a × b × length |
| Capsule | V = π × r² × (length + 2r÷3) |
For the oval tank, a and b are the full width and depth (major and minor axes). The capsule adds two hemispherical end caps to a cylinder body.
Turning volume into gallons and litres
Work the formula in your length unit, then convert once at the end. A tank about a metre in diameter and a metre tall holds roughly 785 litres. Keep every dimension in the same unit before you multiply — mixing metres and centimetres is the fastest way to a wrong answer.
What people get wrong
- Diameter vs radius. Tanks are usually specified by diameter; the cylinder formula needs radius, which is half that. Halving it wrong quadruples the error.
- US vs UK gallons. A US gallon is 3.785 L, a UK gallon is 4.546 L, and an oil barrel is 159 L. On a 40,000 L tank the gap between US and UK gallons is thousands of units.
- Capsule and oval caps. A horizontal cylinder with rounded ends is a capsule, and those end caps add real volume — treating it as a plain cylinder undercounts.
Common questions
How do I calculate the capacity of a cylindrical tank?
Multiply pi by the radius squared by the length or height: V = 3.14159 x r squared x h. A tank with a 0.5 m radius and 2 m height holds 1.57 cubic metres, which is about 1,570 litres.
How do I convert cubic metres to gallons?
One cubic metre is 1,000 litres, 264.2 US gallons, or 220 UK gallons. Multiply your volume in cubic metres by 264.2 for US gallons.
Why does a capsule tank hold more than a plain cylinder?
A capsule has hemispherical caps on each end that add volume beyond the straight cylinder body. The two caps together add the equivalent of a full sphere, roughly a third more than the cylinder alone at short lengths.
