Millimetre
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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
One millimetre is also equal to:
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Since an inch is officially defined as exactly 25.4 millimetres, 1 millimetre is precisely Page Template:Fraction/styles.css has no content.5⁄127 inches (≈ 0.03937 inches).
Definition
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Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of Script error: No such module "Unsubst". of a second".[1]
A millimetre, being Script error: No such module "Unsubst". of a metre, is the distance light travels in Script error: No such module "Unsubst". 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:
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- Square millimetre – Expression error: Unexpected < operatorScript error: No such module "Check for unknown parameters".
- Cubic millimetre – Expression error: Unexpected < operatorScript error: No such module "Check for unknown parameters".[2]
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.[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:
- SI Units – NIST
- Millimetres to inches converter
- Inches to millimetres converter
- International System of Units (BIPM)
- Engineering Toolbox
See also
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References
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