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
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".
A millimetre, being 1/1000 of a metre, is the distance light travels in 1/299 792 458 000 of a second.
Informal terminology
The 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
To 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
These symbols are often used in Japanese typography to align unit symbols with text characters.
Measurement
- On a standard metric ruler, the smallest divisions are typically millimetres.
- Precision engineering rulers may show increments of 0.5 mm.
- Digital calipers often measure to 0.01 mm accuracy.
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 (e.g., 10 Gbps). - The smallest visible object to the human eye is around 0.02–0.04 mm (e.g., a thin human hair). - A typical sheet of paper is between 0.07 mm and 0.18 mm thick; copy paper is about 0.1 mm.
See also
- Metric system
- Orders of magnitude (length)
- Submillimetre astronomy
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