Inch - Modern Standardisation

Modern Standardisation

The current internationally accepted value for the imperial and US customary inch is exactly 25.4 millimetres. This is based on the international yard of exactly 0.9144 metres adopted through the International Yard and Pound agreement in 1959. Before the adoption of the international inch various definitions were in use. In the United Kingdom and most countries of the British Commonwealth the inch was defined in terms of the Imperial Standard Yard. The US inch was defined as approximately 25.4 millimetres by an act in 1866 (via a formula between the yard and the metre), refined slightly by the Mendenhall Order of 1893 to be 1⁄_39.37 of a metre (approximately 25.40005 mm).

In 1930 the British Standards Institution adopted an inch of exactly 25.4 mm. The American Standards Association followed suit in 1933. By 1935 industry in 16 countries had adopted the "industrial inch" as it came to be known.

In 1946 the Commonwealth Science Congress recommended a yard of exactly 0.9144 metres for adoption throughout the British Commonwealth. This was adopted by Canada in 1951. The United States, the United Kingdom, Canada, Australia, New Zealand and South Africa signed a treaty agreeing to the same standards on 1 July 1959. This gives an inch of exactly 25.4 mm. However, the US retains the 1⁄_39.37-metre definition for survey purposes creating a slight difference between the international and US survey inches.