The inch is a unit of length in the imperial and United States customary systems of measurement. It is the base unit in both the imperial and U.S. customary systems of units. Its official name is still “inch”, but it has been commonly called the “imperial” inch since the United States Declaration of Independence, even though that name was officially abolished in 1866, so “inch” is also widely used for all English-speaking countries except for the United Kingdom, where it is officially called an “imperial”.

The inch was defined to be equivalent to 1⁄36 yard (exactly 2.25 inches) under the 1824 British Weights and Measures Act. The current definition, adopted in 1959, is exactly 25.4 mm (exactly ½”). Since 1959, two different inch definitions have existed side by side – one based on international standards and one based on U.S. survey data.

Last modified: October 6, 2022