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A cubic inch (plural: cubic inches) is a non-SI unit of volume, equal to the volume of a cube with sides of one inch.

Cubic inches are still sometimes used as a unit of measurement (in engineering contexts, not household contexts) in the United States, Canada, and United Kingdom, although SI is continuing to gradually displace non-SI usage.

The most common usage is for the capacity of American engines. The UK & Europeans tend to use the Litre for engine capacity.

Wikipedia:Cubic inch for more details