Unit system metric Symbol mm | Unit of length 1 mm in ... ... is equal to ... | |

SI units 6997100000000000000♠1×10 m imperial/US units 0.0032808 ft 0.039370 in |

The **millimetre** (International spelling as used by the International Bureau of Weights and Measures; 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.

## Contents

It is equal to 7003100000000000000♠1000 micrometres or 7006100000000000000♠1000000 nanometres. A millimetre is equal to exactly ^{5}⁄_{127} (approximately 0.039370) 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 1/7008299792458000000♠299792458 of a second". A millimetre, 1/1000 of a metre, is therefore the distance travelled by light in 1/7011299792458000000♠299792458000 of a second.

## Unicode symbols

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

## Measurement

On a metric ruler, the smallest measurements are normally millimetres. 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.

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 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.