Millimetre

millimetre
Ruler with millimetre and centimetre marks
General information
Unit system	SI
Unit of	Length
Symbol	mm
Named after	The metric prefix mille (Latin for "one thousand") and the metre
Conversions
1 mm in ...	... is equal to ...
micrometres	1×103 μm = 1000 μm
centimetres	1×10−1 cm = 0.1 cm
metres	1×10−3 m = 0.001 m
kilometres	1×10−6 km
inches	0.039370 in
feet	0.0032808 ft

The millimetre (international spelling; SI unit symbol mm) or millimeter (American spelling) is a unit of length in the International System of Units (SI), equal to one thousandth of a metre, which is the SI base unit of length. Therefore, there are one thousand millimetres in a metre. There are ten millimetres in a centimetre.

One millimetre is equal to 1000 micrometres or 1000000 nanometres. Since an inch is officially defined as exactly 25.4 millimetres, a millimetre is equal to exactly 5⁄127 (≈ 0.03937) of an inch.

Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of ⁠1/299792458⁠ of a second".^[1] A millimetre, ⁠1/1000⁠ of a metre, is therefore the distance travelled by light in ⁠1/299792458000⁠ of a second.

A common shortening of millimetre in spoken English is "mil". This can cause confusion since in the United States, "mil" traditionally means a thousandth of an inch.

For the purposes of compatibility with Chinese, Japanese and Korean (CJK) characters, Unicode has symbols for:^[2]

• millimetre - U+339C ㎜ SQUARE MM
• square millimetre - U+339F ㎟ SQUARE MM SQUARED
• cubic millimetre U+33A3 ㎣ SQUARE MM CUBED

In Japanese typography, these square symbols are used for laying out unit symbols without distorting the grid layout of text characters.

On a metric ruler, the smallest measurements are normally millimetres.^[3] High-quality engineering rulers may be graduated in increments of 0.5 mm. Digital callipers are commonly capable of reading increments as small as 0.01 mm.^[4]

Microwaves with a frequency of 300 GHz have a wavelength of 1 mm. Using frequencies between 30 GHz and 300 GHz for data transmission, in contrast to the 300 MHz to 3 GHz normally used in mobile devices, has the potential to allow data transfer rates of 10 gigabits per second.^[5]

The smallest dimension the human eye can resolve is around 0.02 to 0.04 mm, approximately the width of a thin human hair.^[6] A sheet of paper is typically between 0.07 mm and 0.18 mm thick, with ordinary printer paper or copy paper approximately 0.1 mm thick.^[7]

Look up millimetre in Wiktionary, the free dictionary.

• Metric system
• Orders of magnitude (length)
• Submillimetre astronomy