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An inch (plural: inches; abbreviation or symbol: in or ″ – a double prime) is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot. A corresponding unit of area is the square inch and a corresponding unit of volume is the cubic inch. The inch is one of the most common units of length in the United States, and is widely used also in the United Kingdom, and Canada, despite the introduction of metric to the latter two in the 1960s and 1970s, respectively. The inch is sometimes used informally in other Commonwealth nations, such as Australia.


International inch

Effective July 1, 1959, the United States and countries of the British Commonwealth defined the length of the international yard to be exactly 0.9144 meters.[1] Consequently, the international inch is defined to be equal to exactly 25.4 millimeters. This creates a slight difference between the international units and surveyor's units which are described in the article on the foot.

The international standard symbol for inch is in (see ISO 31-1, Annex A). In some cases, the inch is denoted by a double prime, which is often approximated by double quotes, and the foot by a prime, which is often approximated by an apostrophe.

Equivalence to other units of length

1 international inch is equal to:

  • 1,000 thou (1 thou is 0.001 inches.)
  • 1,000 mil (1 mil = 1 thou = 0.001 inches)
  • 1,000,000 microinches (1 μin is one millionth of an inch.)
  • ≈ 0.02778 yards (1 yard is equal to 36 inches.)
  • 2.54 centimeters (1 centimeter ≈ 0.3937 international inches.)

Historical origin

Full article ▸

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