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

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:

• 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".^[1]

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^[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 (e.g., 10 Gbps).^[5]
• The smallest visible object to the human eye is around 0.02–0.04 mm (e.g., 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]

See also

Look up millimetre in Wiktionary, the free dictionary.

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

References