Gallons to Cubic Inches Converter

The Conversion Factor

One US liquid gallon equals exactly 231 cubic inches — the legal definition of the US gallon, not a rounded figure. The tool multiplies your input by 231: 5 gallons returns 1155 cubic inches, and 0.5 gallons returns 115.5.

Why 231 Specifically

The US gallon was historically defined as the volume of a cylinder of a fixed diameter and height, and 231 cubic inches is what that works out to once standardized. This is the US gallon specifically — the UK imperial gallon is defined differently and equals about 277.4 cubic inches, so don't use this converter for imperial gallon figures.

Practical Use

This conversion comes up when sizing tanks and containers, or with engine displacement figures — cubic inches was the standard US way to describe engine size for decades (a "350" engine means 350 cubic inches), and you may need to relate that back to a gallon-based capacity spec.