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Ruler with millimeter and centimeter marks.png
Ruler with millimetre and centimetre marks
General information
Unit system SI derived unit
Unit of Length
Named afterThe metric prefix mille (Latin for "one thousand") and the metre
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

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. Therefore, there are one thousand millimetres in a metre. There are ten millimetres in a centimetre.


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.


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:

In Japanese typography, these square symbols were historically[ when? ] used for laying out unit symbols without distorting the grid layout of text characters.


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 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]

See also

Related Research Articles

Centimetre unit of length equal to 1/100 of a metre

A centimetre or centimeter is a unit of length in the metric system, equal to one hundredth of a metre, centi being the SI prefix for a factor of 1/100. The centimetre was the base unit of length in the now deprecated centimetre–gram–second (CGS) system of units.

The inch is a unit of length in the (British) imperial and United States customary systems of measurement. It is equal to ​136 yard or ​112 of a foot. Derived from the Roman uncia ("twelfth"), the word inch is also sometimes used to translate similar units in other measurement systems, usually understood as deriving from the width of the human thumb.

Litre unit of volume accepted for use with the SI

The litre or liter is a non-SI unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre. A cubic decimetre occupies a volume of 10 cm × 10 cm × 10 cm and is thus equal to one-thousandth of a cubic metre.

Metre SI unit of length

The metre or meter is the base unit of length in the International System of Units (SI). The SI unit symbol is m. The metre is defined as the length of the path travelled by monochromatic light in a vacuum in 1/299 792 458 of a second. The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's circumference is approximately 40000 km. In 1799, the metre was redefined in terms of a prototype metre bar. In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted.

Micrometre one millionth of a metre

The micrometre or micrometer, also commonly known by the previous deprecated name micron, is an SI derived unit of length equalling 1×10−6 metre ; that is, one millionth of a metre.

A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or fraction of the unit. While all metric prefixes in common use today are decadic, historically there have been a number of binary metric prefixes as well. Each prefix has a unique symbol that is prepended to the unit symbol. The prefix kilo-, for example, may be added to gram to indicate multiplication by one thousand: one kilogram is equal to one thousand grams. The prefix milli-, likewise, may be added to metre to indicate division by one thousand; one millimetre is equal to one thousandth of a metre.

Micrometer device incorporating a calibrated screw

A micrometer, sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw widely used for accurate measurement of components in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier, and digital calipers. Micrometers are usually, but not always, in the form of calipers. The spindle is a very accurately machined screw and the object to be measured is placed between the spindle and the anvil. The spindle is moved by turning the ratchet knob or thimble until the object to be measured is lightly touched by both the spindle and the anvil.

Pascal (unit) SI unit of pressure

The pascal is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus and ultimate tensile strength. The unit, named after Blaise Pascal, is defined as one newton per square metre. The unit of measurement called standard atmosphere (atm) is defined as 101325 Pa.

The metre per second is an SI derived unit of both speed and velocity, equal to the speed of a body covering a distance of one metre in a time of one second.

Cubic centimetre unit of volume

A cubic centimetre is a commonly used unit of volume that corresponds to the volume of a cube that measures 1 cm × 1 cm × 1 cm. One cubic centimetre corresponds to a volume of one millilitre. The mass of one cubic centimetre of water at 3.98 °C is closely equal to one gram.

Square metre SI-derived unit of area

The square metre or square meter is the SI derived unit of area with symbol m2.

In the metric system, a microgram or microgramme is a unit of mass equal to one millionth of a gram. The unit symbol is μg according to the International System of Units; the recommended symbol in the United States and United Kingdom when communicating medical information is mcg. In μg the prefix symbol for micro- is the Greek letter μ (Mu).

Bar (unit) non-SI unit of pressure

The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), which is the atmospheric pressure on earth at an altitude of about 111 meters and a temperature of 15 °C or slightly less than the current average pressure at sea level.

Unit of length Reference value of length

A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are U.S. customary units in the United States and metric units elsewhere. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.

Tape measure Flexible ruler used to measure size or distance

A tape measure or measuring tape is a flexible ruler used to measure size or distance.

Milliradian unit of of angular measurement used for adjustment of firearm sights and range estimation

A milliradian is an SI derived unit for angular measurement which is defined as a thousandth of a radian. Milliradians are used in adjustment of firearm sights by adjusting the angle of the sight compared to the barrel. Milliradians are also used for comparing shot groupings, or to compare the difficulty of hitting different sized shooting targets at different distances. When using a scope with both mrad adjustment and a reticle with mrad markings, the shooter can use the reticle as a "ruler" to count the number of mrad's a shot was off target, which directly translates to the sight adjustment needed to hit the target with a follow up shot. Optics with mrad markings in the reticle can also be used to make a range estimation of a known size target, or vice versa, to determine a target size if the distance is known, a practice called "milling".

International standard ISO 2848 is an ISO standard used by the construction industry. It is based on multiples of 300 mm and 600 mm

A thousandth of an inch is a derived unit of length in a system of units using inches. Equal to ​11000 of an inch, it is normally referred to as a thou, a thousandth, or a mil.

Angstrom Unit of length; equals 0.1 nanometre

The angstrom or ångström is a metric unit of length equal to 10−10 m; that is, one ten-billionth of a metre, 0.1 nanometre, or 100 picometres. Its symbol is Å, a letter of the Swedish alphabet.

Cubic metre SI unit of volume

The cubic metre or cubic meter is the SI derived unit of volume. Its SI symbol is m3. It is the volume of a cube with edges one metre in length. An alternative name, which allowed a different usage with metric prefixes, was the stère, still sometimes used for dry measure. Another alternative name, no longer widely used, was the kilolitre.


  1. "17th General Conference on Weights and Measures (1983), Resolution 1". International Bureau of Weights and Measures. Retrieved 3 December 2013.
  2. 1 2 3 "CJK Compatibility" (PDF). Retrieved 3 December 2013.
  3. "How do I read a ruler?". Retrieved 3 December 2013.
  4. "Accuracy of Calipers". Retrieved 3 December 2013.
  5. Huang, Kao-Cheng; Wang, Zhaocheng (2011). Millimeter Wave Communication Systems. ISBN   9781118102756.
  6. "How Small Can the Naked Eye See?". Focus Magazine. Retrieved 3 December 2013.
  7. "Thickness of a Piece of Paper". Archived from the original on 8 June 2017. Retrieved 3 December 2013.