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Cubic inches calculator
Get the exact cubic inches of a cylinder and see it converted to US gallons using the 231 in³ rule.
For engine work, small tanks and precise jobs, cubic inches is the natural unit, and this cubic inches calculator delivers it. It also does the step every gearhead needs — dividing by 231 to turn in³ into US gallons.
How the calculation works
- Measure across the circular end — the diameter or radius.
- If you measured the diameter, halve it to get the radius r.
- Square the radius and multiply by π to get the base area.
- Multiply the base area by the height h: V = π r² h.
- Convert the result into your preferred unit above.
Worked example. Take a cylinder with a diameter of 3 in (so a radius of 1.5 in) and a height of 8 in. The base area is π × 1.5² = 7.07 in², and multiplying by the height gives a volume of about 0.2 US gallons (57 in³ · 0.9 litres).
| Unit | Equal to |
|---|---|
| 1 US gallon | 231 in³ · 3.785 L |
| 1 Imperial gallon | 277.42 in³ · 4.546 L |
| 1 cubic foot | 7.481 US gal · 28.32 L |
| 1 litre | 0.264 US gal · 0.220 Imp gal |
| 1 US gallon | 128 US fl oz |
For anything that holds liquid, measure the inside of the cylinder — the wall takes up space that the contents cannot.
Every cubic inches page
Each page below targets a specific question and opens with the calculator ready to go:
Good to know
What is the formula for the volume of a cylinder?
V = π r² h — the area of the circular base (π r²) multiplied by the height (h). If you have the diameter instead of the radius, halve it first: r = d ÷ 2.