Convert inches to mm (2024)

Please provide values below to convert inch [in] to millimeter [mm], or vice versa.

Inch

Definition: An inch (symbol: in) is a unit of length in the imperial and US customary systems of measurement. An inch was defined to be equivalent to exactly 25.4 millimeters in 1959. There are 12 inches in a foot and 36 inches in a yard.

History/origin: The term "inch" was derived from the Latin unit "uncia" which equated to "one-twelfth" of a Roman foot.

There have been a number of different standards for the inch in the past, with the current definition being based on the international yard. One of the earliest definitions of the inch was based on barleycorns, where an inch was equal to the length of three grains of dry, round barley placed end-to-end. Another version of the inch is also believed to have been derived from the width of a human thumb, where the length was obtained from averaging the width of three thumbs: a small, a medium, and a large one.

Current use: The inch is mostly used in the United States, Canada, and the United Kingdom. It is also sometimes used in Japan (as well as other countries) in relation to electronic parts, like the size of display screens.

Millimeter

Definition: A millimeter (symbol: mm) is a unit of length in the International System of Units (SI). It is defined in terms of the meter, as 1/1000 of a meter, or the distance traveled by light in 1/299 792 458 000 of a second.

History/origin: The milli- prefix is one of many metric prefixes. It indicates one thousandth of the base unit, in this case the meter. The definition of the meter has changed over time, the current definition being based on the distance traveled by the speed of light in a given amount of time. The relationship between the meter and the millimeter is constant however. Prior to this definition, the meter was based on the length of a prototype meter bar. In 2019, the meter has been re-defined based on the changes made to the definition of a second.

Inch to Millimeter Conversion Table

How to Convert Inch to Millimeter

1 in = 25.4 mm
1 mm = 0.0393700787 in

Example: convert 15 in to mm:
15 in = 15 × 25.4 mm = 381 mm

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