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A cubic inch 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 and Canada, although SI is continuing to gradually displace non-SI usage.

Notation conventions

The following symbols are used to denote cubic inches:

cubic in
cu inches, cu inch, cu in
inches/-3, inch/-3, in/-3
inches^3, inch^3, in^3
inches³ ,inch³, in³
c.i., CI
c.i.d., cid, CID —for cubic inch displacement in internal combustion engines

Equivalence with other units of volume

1 cubic inch (assuming an international inch) is equal to:

0.000578703703703 cubic feet (1 cu ft equals 1,728 cu in)
about 0.554112552 U.S. fluid ounces
about 0.069264069 U.S. cups
about 0.000465025413 U.S. bushels
about 0.004329 U.S. liquid gallons (1 gallon equals 231 cu in exactly (3 in × 7 in × 11 in))
about 0.00010307 crude barrels (1 barrel equals 42 gallons, or 9702 cu in)
exactly 0.016387064 litres (1 L is about 61 cu in [exactly 61.0237441 cu in])
exactly 16.387064 millilitres or cubic centimetres (which in turn are about 0.061 cu in)
exactly 0.000016387064 cubic metres (1 m³ is about 61,023.75 cu in)

Uses of the cubic inch

Electrical box volume

The cubic inch was established decades ago as the conventional unit in the U.S. for measuring the volume of electrical boxes; SI has not yet replaced it for this purpose.

Engine displacement

The cubic inch was formerly used (until the 1980s) to express the nominal size (displacement) of engines (e.g. 426 HEMI) for new cars, trucks, etc. It is therefore still used for this purpose in the context of the classic-car hobby, auto racing, and so forth. The auto industry nowadays uses SI for this purpose (e.g. 6.1 L HEMI). For more information and a list of CID-to-liter conversions, see engine displacement.