330px, Different lengths as in respect of the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 metre to 1 millimetre.
The millimetre (American and British English spelling differences#-re, -er, international spelling; International System of Units, SI unit symbol mm) or millimeter (American and British English spelling differences#-re, -er, American spelling) is a
Units of measurement, unit of
length
Length is a measure of distance. In the International System of Quantities, length is a quantity with Dimension (physical quantity), dimension distance. In most systems of measurement a Base unit (measurement), base unit for length is chosen, ...
in the
International System of Units
The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...
(SI),
equal to one thousandth of a
metre
The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...
, which is the
SI base unit
The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which al ...
of length. Therefore, there are one thousand millimetres in a metre, and there are ten millimetres in a centimetre.
One millimetre is equal to
micrometre
The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...
s or
nanometres.
Since an
inch
The inch (symbol: in or prime (symbol), ) is a Units of measurement, unit of length in the imperial units, British Imperial and the United States customary units, United States customary System of measurement, systems of measurement. It is eq ...
is officially defined as exactly 25.4 millimetres, a millimetre is equal to exactly (≈ 0.03937) of an inch.
Definition
Since 1983, the
metre
The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...
has been defined as "the length of the path travelled by
light
Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
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.
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
CJK compatibility with
Chinese,
Japanese and
Korean (
CJK) characters,
Unicode
Unicode or ''The Unicode Standard'' or TUS is a character encoding standard maintained by the Unicode Consortium designed to support the use of text in all of the world's writing systems that can be digitized. Version 16.0 defines 154,998 Char ...
has
form symbols for:
* millimetre
* square millimetre
* cubic millimetre
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
Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, Textile, rags, poaceae, grasses, Feces#Other uses, herbivore dung, or other vegetable sources in water. Once the water is dra ...
is typically between 0.07 mm and 0.18 mm thick, with ordinary printer paper or copy paper approximately 0.1 mm thick.
See also
*
Metric system
The metric system is a system of measurement that standardization, standardizes a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules gover ...
*
Orders of magnitude (length)
*
Submillimetre astronomy
References
{{SI units of length
Metre
-03
1000 (number)