Ruler with millimetre and centimetre marks
|Unit system||SI derived unit|
|Named after||The metric prefix mille (Latin for "one thousand") and the metre|
|micrometres||= 1000 ?m|
|centimetres||cm = 0.1 cm|
|metres||m = 0.001 m|
The millimetre (international spelling; SI unit symbol mm) or millimeter (American spelling) is a unit of length in the metric system, 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.
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". 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.
In Japanese typography, these square symbols were historically[when?] used for laying out unit symbols without distorting the grid layout of text characters.
On a metric ruler, the smallest measurements are normally millimetres. 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.
Microwaves with a frequency of 300 GHz have a wavelength of 1 mm. Using wavelengths 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.
The smallest distances the human eye can resolve is around 0.02 to 0.04 mm, approximately the width of a thin human hair. A sheet of paper is typically between 0.07 mm and 0.18 mm thick, with ordinary printer paper or copy paper approximately a tenth of a millimetre thick.