# Millimetre

(Redirected from Milimeter)
1 millimetre =
SI units
1×10−3 m 0.1 cm
US customary units (Imperial units)
3.2808×10−3 ft 39.370×10−3 in
Further information: Metre

The millimetre (International spelling as used by the International Bureau of Weights and Measures) or millimeter (American spelling) (SI unit symbol mm) is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length.

It is equal to 1,000 micrometres and 1,000,000 nanometres. There are 25.4 mm in one inch by definition, so a millimetre is exactly equal to 5127 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."[1] A millimetre, 1/1000 of a metre, is therefore the distance travelled by light in 1/299,792,458,000 of a second.

## Unicode symbols

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

• millimetre (㎜) - code U+339C[2]
• square millimetre (㎟) - code U+339F[2]
• cubic millimetre (㎣) - code U+33A3[2]

## Measurement

On a metric ruler, the smallest measurements are normally millimetres.[3] High-quality engineering rules may be graduated in increments of 0.5 mm. Digital Vernier 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 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.[5]

The smallest distances the human eye can resolve is around 0.02 to 0.04 mm, approximately the width of a 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 a tenth of a millimetre thick.[7]