The inch is a subsidiary unit of length in the British Imperial system, defined as 1/36 yard.

The British Imperial system of units was established on June 17, 1824, by the Weights and Measures Act. Strictly speaking, it is not correct to use the term British Imperial for units in use before that date, and they are better described as traditional British. In this system, the standard yard had a length of 0.914398416 m[1], making the inch equal to 0.025399956 m. (Cardarelli[2] gives a value of 0.91443992 m, but this is in error.) For the detailed history of this yard, see Yard (British Imperial).

In 1963, a new Weights and Measures Act defined the yard in terms of the metric units, so that the value has since then agreed with the International yard of 0.9144 m, slightly larger than the earlier value (but smaller than the value current in the United States prior to unification), making the inch equal exactly to 0.0254 m, conforming to an agreement with the United States.


  1. Sears et al., Phil. Trans. A 227:281 (1928)
  2. Cardarelli, François (1998). Scientific Unit Conversion. Berlin, Heidelberg, New York: Springer-Verlag. ISBN 3-540-76022-9. 

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