A US gallon is defined as exactly 231 cubic inches, making this a precise volume conversion in both directions.
How it works
Dividing cubic inches by 231 gives gallons; multiplying gallons by 231 gives cubic inches back.
What this does not include
This does not include cubic feet — for that volume unit, use this site’s cubic feet to gallons calculator instead.
How to use this calculator
- Enter a value in either cubic inches or gallons — the other field fills in automatically.
Frequently asked questions
Why is 231 the exact conversion factor?
Because the US gallon is legally defined as exactly 231 cubic inches — it’s not an approximation, but the gallon’s actual definition.
Where is this conversion commonly needed?
Engine displacement specs, fuel tank capacity, and container sizing often start in cubic inches and need converting to gallons for practical use.
Is this the same 231 constant used elsewhere on this site?
Yes — the same exact gallon definition underlies the cubic feet to gallons converter elsewhere on the site.
