The millimetre (SI symbol: mm; international spelling) or millimeter (American spelling) is a unit of length in the International System of Units (SI), equal to one thousandth of a metre, the SI base unit of length.
- 1 metre = 1000 millimetres
- 1 centimetre = 10 millimetres
| millimetre | |
|---|---|
 Ruler with millimetre and centimetre marks | |
| General information | |
| Unit system | SI |
| Unit of | Length |
| Symbol | mm |
| Named after | From metric prefix mille (Latin for "one thousand") and the metre |
| Conversions | |
| 1 mm in ... | ... is equal to ... |
| micrometres | 1000 |
| centimetres | 1/10 |
| metres | 1/1000 |
| kilometres | 1/1 000 000 |
| decimetres | 1/100 |
| inches | 0.039370 in |
| feet | 0.0032808 ft |
One millimetre is also equal to:
- 1000 micrometres
- 1000000 nanometres
Since an inch is officially defined as exactly 25.4 millimetres, 1 millimetre is precisely 5⁄127 inches (≈ 0.03937 inches).
Definition
editInformal terminology
editThe term "mil" is sometimes used colloquially for millimetre. However, in the United States, "mil" traditionally means a thousandth of an inch, which may cause confusion.
Unicode symbols
editTo support layout compatibility with East Asian scripts (CJK), Unicode includes square symbols for:
- Millimetre – U+339C ㎜ SQUARE MM
- Square millimetre – U+339F ㎟ SQUARE MM SQUARED
- Cubic millimetre – U+33A3 ㎣ SQUARE MM CUBED[2]
These symbols are often used in Japanese typography to align unit symbols with text characters.
Measurement
edit- On a standard metric ruler, the smallest divisions are typically millimetres.[3]
- Precision engineering rulers may show increments of 0.5 mm.
- Digital calipers often measure to 0.01 mm accuracy.[4]
Examples:
- Microwaves with a frequency of 300 GHz have a wavelength of 1 mm.
- Using frequencies from 30–300 GHz for millimetre-wave communications allows high-speed data transfer, such as 10 Gbps.[5]
- The smallest visible object to the human eye is around 0.02–0.04 mm, such as a thin human hair.[6]
- A typical sheet of paper is between 0.07 mm and 0.18 mm thick; copy paper is about 0.1 mm.[7]
- Mechanical pencil leads are commonly produced in 0.5 mm and 0.7 mm sizes.
- Printed circuit boards, automotive components and precision-machined engineering parts are frequently specified using millimetres.
Millimetres are also commonly converted into imperial units such as inches for engineering, manufacturing, construction and international product specifications. One inch is defined as exactly 25.4 millimetres.[8][9]
Additional metric and imperial conversion references:
See also
edit
References
edit- ↑ "17th General Conference on Weights and Measures (1983), Resolution 1". International Bureau of Weights and Measures. Retrieved 3 December 2013.
- ↑ "CJK Compatibility" (PDF). unicode.org. Retrieved 3 December 2013.
- ↑ "How do I read a ruler?". onlineconversion.com. Retrieved 3 December 2013.
- ↑ "Accuracy of Calipers". TresnaInstrument.com. Retrieved 3 December 2013.
- ↑ Huang, Kao-Cheng; Wang, Zhaocheng (2011). Millimeter Wave Communication Systems. John Wiley & Sons. ISBN 9781118102756.
- ↑ "How Small Can the Naked Eye See?". Focus Magazine. Retrieved 3 December 2013.
- ↑ Sherlis, Juliya (2001). Elert, Glenn (ed.). "Thickness of a piece of paper". The Physics Factbook. Retrieved 2022-01-21.
- ↑ "SI Units – Length". National Institute of Standards and Technology. Retrieved 24 May 2026.
- ↑ "Millimetres to Inches Conversion Reference". WhatsThisIn. Retrieved 24 May 2026.