Millimeters to Inches (mm to in)

1 millimeter equals 0.03937007874 inches (rounded).

1 millimeter = 0.03937007874 inches

How to convert millimeters to inches

To convert millimeters to inches, multiply the number of millimeters by 0.03937007874. The factor is rounded to 10 significant digits. To go the other way, multiply inches by 25.4.

inches = millimeters × 0.03937007874

Example: 10 millimeters = 0.3937007874 inches.

About these units

Millimeter (mm)

Metric (SI)Current unit

One thousandth of a meter. Used in engineering, manufacturing and rainfall measurement.

Carl Friedrich Gauss put the millimeter at the base of his 1832 proposal for absolute units, pairing it with the milligram and the second, although the centimeter-based CGS system later won out among scientists. The name combines the metric prefix milli-, from the Latin for one thousand, with the meter. Since 1983 a millimeter has been tied to the speed of light: it is the distance light covers in 1/299,792,458,000 of a second.

Millimeters are the language of fine mechanical work. Standard rulers are divided into them, digital calipers commonly read to 0.01 mm, and mechanical pencil leads come in 0.5 and 0.7 mm. Copy paper is about 0.1 mm thick, and a thin hair, at 0.02 to 0.04 mm across, is close to the smallest thing the naked eye can see. A wave with a frequency of 300 GHz has a wavelength of 1 mm. An inch is exactly 25.4 mm. One trap for readers in the United States is that mil there traditionally means a thousandth of an inch, not a millimeter.

1 millimeter = 0.001 meters (exactly).

Sources: Wikipedia: Millimetre; Wikipedia: Centimetre-gram-second system of units; NIST Guide to the SI, Appendix B.8: conversion factors

Inch (in)

US & ImperialCurrent unit

1/12 of a foot. Defined as exactly 25.4 millimeters since 1959, when the US and Commonwealth countries agreed a common international inch.

Latin uncia, meaning one-twelfth, is the root of both inch and ounce, and it passed into Old English as ynce. Roman measurers divided their foot into twelve unciae, and the English inch kept that twelfth. After 1066 an inch was reckoned as three barleycorns, and a 1324 statute attributed to Edward II spelled out the definition as three grains of barley, dry and round, placed end to end lengthwise. For centuries the inch was simply a twelfth of a foot, or a thirty-sixth of a yard, and the US and UK each kept their own copy of the standard yard.

Those copies differed slightly until the 1959 international agreement fixed the yard at exactly 0.9144 meter, which made the inch exactly 25.4 millimeters. The US adopted it in 1959 and the UK ratified it in 1963, effective 1964. Today the inch is still the customary unit in the United States and Canada, and the UK keeps it on road signs. Worldwide it also names display diagonals, wheel rims, tire codes and pipe sizes.

1 inch = 0.0254 meters (exactly).

Sources: Wikipedia: Inch; Wikipedia: Yard; NIST Guide to the SI, Appendix B.8: conversion factors

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