Cubic inches
Cubic Inches of Cylinder
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 of cylinder delivers it. It also does the step every gearhead needs — dividing by 231 to turn in³ into US gallons.
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.
What people work out next
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.
Where people go wrong
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
Showing every unit at once is about speed and safety: you see the answer in litres, gallons and cubic feet side by side, so there is no chance of applying the wrong conversion factor. Change the output unit at the top to make any of them the headline figure, switch the precision to exact if you want the value in terms of pi, and use copy-result to paste a tidy summary wherever you need it.
What to work out next
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
It comes in handy across quite different tasks:
- Blueprinting an engine's displacement
- Sizing a small precision reservoir
- Converting a machined bore to gallons
To sum up, the whole job is two measurements and one formula. Enter them above, pick your unit, and you have an accurate figure plus every equivalent unit beside it. Keep a copy with the copy-result button, save the shareable link if someone else needs the same numbers, and use the history panel to compare a few options without starting from scratch.
Frequently asked questions
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.