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 1,000 micrometres or 1,000,000 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.

Definition

Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second". A millimetre, 1⁄1000 of a metre, is therefore the distance travelled by light in 1⁄299,792,458,000 of a second.

Informal terminology

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

Unicode symbols

For the purposes of compatibility with Chinese, Japanese and Korean (CJK) characters, Unicode has symbols for: In Japanese typography, these square symbols are used for laying out unit symbols without distorting the grid layout of text characters.

Measurement

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 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. The smallest dimension 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 0.1 mm thick.