- See also: U.S. customary units
The inch (abbreviation: in and often the double prime " ) is a unit of length in the Imperial and the U.S. customary systems of measurement. An inch is equal to 1,000 mil, and twelve inches are equal to one foot.
The inch was originally derived from the breadth of a man's thumb, although its precise definition has changed over the years as the two systems have evolved. The inch used today is the international inch, which is equal to 1/12 of a foot, or 2.54 centimetres.
The inch is equal to: