National Physical Laboratory

On what basis is one inch exactly equal to 25.4 mm? Has the imperial inch been adjusted to give this exact fit and if so when? (FAQ - Length)

The (international) inch has been exactly 25.4 mm since July 1959. At this point in time the (international) yard was redefined as 0.9144 metre - until this time the ratio between the US yard and the metre was different to the ratio between the UK yard and the metre. For more information, see Engineering Metrology by K J Hume (2 ed) Macdonald London 1967. The American inch changed by 2 millionths of an inch and the UK inch by 1.7 millionths of an inch. The international inch falls mid way between the old UK and US inch.


  • The Yard Unit of Length, Nature, Vol 200, No 4908 pp 730-732 23 Nov 1963
  • The United Kingdom standards of the yard in terms, of the metre (British Applied Journal of Physics)
Last Updated: 23 Sep 2015
Created: 17 Jul 2007


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