How the Inch Became Exactly 25.4 Millimetres

By the twentieth century both Britain and the United States defined their customary units in relation to the metre, but not identically. The 1893 Mendenhall Order had made the US yard 3600/3937 of a metre, which gave an inch of about 25.4000508 mm, while the British inch worked out slightly shorter. For most purposes the gap of a few parts per million was invisible, but precision engineering and wartime production of interchangeable parts made it a nuisance.

Industry had already adopted 25.4 mm as a convenient industrial inch in the 1930s. In 1959 the national standards laboratories of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa signed the international yard and pound agreement, defining the yard as exactly 0.9144 metres and the avoirdupois pound as exactly 0.45359237 kilograms. The inch therefore became exactly 25.4 mm and the foot exactly 0.3048 m.

American land surveyors were reluctant to change the coordinates of millions of records, so the old foot survived as the US survey foot of 1200/3937 m. The difference, two parts per million, amounts to about 3.2 millimetres per mile, enough to matter over state plane coordinate systems. After decades of confusion, NIST and NOAA deprecated the survey foot from 1 January 2023.

The 1959 values are why every imperial and US customary length and weight can be expressed as an exact decimal number of metres or kilograms: the acre is exactly 4046.8564224 square metres, and the grain exactly 64.79891 milligrams.