# Millimetre

(Redirected from 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.

millimetre
Ruler with millimetre and centimetre marks
General information
Unit systemSI
Unit ofLength
Symbolmm
Named afterThe 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

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 5127 (≈ 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 1/299792458 of a second".[1] A millimetre, 1/1000 of a metre, is therefore the distance travelled by light in 1/299792458000 of a second.

## Informal terminology

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.

## Unicode symbols

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.

## Measurement

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]