The inch (symbol: in or ) is a unit of length in the British imperial and the United States customary systems of measurement. It is equal to 1/36 yard or 1/12 of a foot. Derived from the Roman uncia ("twelfth"), the word inch is also sometimes used to translate similar units in other measurement systems, usually understood as deriving from the width of the human thumb.

Unit systemImperial/US units
Unit ofLength
Symbolinor″ (the double prime)[1]
1 in in ...... is equal to ...
   Imperial/US units   1/36 yd or 1/12 ft
   Metric (SI) units   25.4 mm
Measuring tape with inches
A fire hydrant marked as 3-inch

Standards for the exact length of an inch have varied in the past, but since the adoption of the international yard during the 1950s and 1960s the inch has been based on the metric system and defined as exactly 25.4 mm.

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This article uses material from the Wikipedia article Inch, and is written by contributors. Text is available under a CC BY-SA 4.0 International License; additional terms may apply. Images, videos and audio are available under their respective licenses.