Millimetre

 ( Metre ) 

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: - micrometres - nanometres

Since an inch is officially defined as exactly 25.4 millimetres, 1 millimetre is precisely inches (≈ 0.03937 inches).

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Definition Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of of a second".

A millimetre, being of a metre, is the distance light travels in of a second.

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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.

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Unicode symbols To support layout compatibility with East Asian scripts (CJK), Unicode includes square symbols for:

• Millimetre –
• Square millimetre –
• Cubic millimetre –

These symbols are often used in Japanese typography to align unit symbols with text characters.

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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.

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See also

• Metric system
• Orders of magnitude (length)
• Submillimetre astronomy

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