| millimetre | |
| Standard: | SI |
| Quantity: | Length |
| Symbol: | mm |
| Namedafter: | The metric prefix mille (Latin for "one thousand") and the metre |
| Units1: | micrometres |
| Inunits1: | = 1000 μm |
| Units2: | centimetres |
| Inunits2: | cm = 0.1 cm |
| Units3: | metres |
| Units4: | kilometres |
| Units5: | inches |
| Inunits5: | 1mm |
| Units6: | feet |
| Inunits6: | 1mm |
thumb|330px|Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter.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 micrometres or nanometres.Since an inch is officially defined as exactly 25.4 millimetres, a millimetre is equal to exactly (≈ 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 of a second".[1] A millimetre, of a metre, is therefore the distance travelled by light in 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]
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]