Cubic inches
Cubic Inches in a Cylinder Calculator
Get the exact cubic inches of a cylinder and see it converted to US gallons using the 231 in³ rule.
This cubic inches in a cylinder calculator returns the exact cubic-inch volume of a cylinder and shows the gallon conversion right beside it, because 231 cubic inches is one US gallon and that number comes up constantly.
Cubic inches is the language of engines and precise, smaller work. It is the unit behind the classic how-many-cubic-inches of a bore, and it converts to US gallons through one exact number, 231, which is worth committing to memory if you handle tanks in imperial units.
The formula behind it
The volume of a cylinder is:
V = π r² hA worked example
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.
Conversions use standard factors: 1 US gallon = 231 in³, 1 Imperial gallon = 4.54609 L, 1 litre = 1000 cm³.
How to work it out
- 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.
Getting an accurate measurement
If you can only reach the top, measure the diameter across the opening and halve it. Keep every measurement in the same system, and let the tool convert between units for you.
Follow-on questions, answered
To save you a second search, here are the related answers people most often want straight after this calculation:
What is it in cc (cubic centimetres)?
One cubic inch is about 16.39 cc, so the cm3 figure in the breakdown is the metric version that engine builders usually quote.
How do I turn cubic inches into gallons?
Divide by 231 for US gallons — that is the exact definition of a US gallon — or by about 277.42 for Imperial. Both gallon values are shown with the result.
Is this the same as engine displacement?
For a single cylinder, yes: the bore radius and the stroke give the swept volume in cubic inches. Multiply by the number of cylinders for total displacement.
Common mistakes to avoid
The commonest slip is using the diameter where the formula wants the radius, which on its own quadruples the base area. Close behind is mixing units, such as a radius in centimetres with a height in metres. Keep every length in the same unit, or let the tool convert, and double-check whether you measured to the centre or across the whole width. When a figure looks surprising, that width measurement is almost always where the problem is hiding, so re-read it first.
Reading and converting your result
Because volume units are where most errors creep in, the tool does the converting for you. Enter your measurements once and every common unit appears together, from millilitres up to cubic metres and across to US and Imperial gallons. Pick whichever the task calls for and lift the rest from the list; nothing needs a second calculation. The result also updates live as you change a measurement, so you can compare two sizes quickly without starting over.
Related things you might need
Once you have your answer you may also want to work in gallons cubic feet volume formula. These related calculators pick up where this one leaves off:
Common uses
A few of the jobs it gets used for:
- Blueprinting an engine's displacement
- Sizing a small precision reservoir
- Converting a machined bore to gallons
The bottom line: with the radius or diameter and the height, the volume follows immediately, and the breakdown lets you hand the same number to anyone in the unit they prefer. Measure inside dimensions for anything that holds liquid, leave a little headspace at the top of a real container, and treat the geometric figure as a clean maximum rather than the final word on usable capacity.
Questions people ask
Does this work for a tank or a pipe?
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.