The micrometre (Commonwealth English) as used by the International Bureau of Weights and Measures;^[1] SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron,^[2] is a unit of length in the International System of Units (SI) equalling 1×10⁻⁶ metre (SI standard prefix "micro-" = 10⁻⁶); that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.00004 inch).^[1]

The nearest smaller common SI unit is the nanometre, equivalent to one thousandth of a micrometre, one millionth of a millimetre or one billionth of a metre (0.000000001 m).

The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria,^[1] and for grading wool by the diameter of the fibres.^[3] The width of a single human hair ranges from approximately 20 to 200 μm.

Between 1 μm and 10 μm:

• 1–10 μm – length of a typical bacterium
• 3–8 μm – width of strand of spider web silk^[4]
• 5 μm – length of a typical human spermatozoon's head^[5]
• 10 μm – size of fungal hyphae
• about 10 μm – size of a fog, mist, or cloud water droplet

Between 10 μm and 100 μm:

• about 10–12 μm – thickness of plastic wrap (cling wrap)
• 10 to 55 μm – width of wool fibre^[6]
• 17 to 181 μm – diameter of human hair^[7]
• 70 to 180 μm – thickness of paper

The term micron and the symbol μ were officially accepted for use in isolation to denote the micrometre in 1879, but officially revoked by the International System of Units (SI) in 1967.^[8] This became necessary because the older usage was incompatible with the official adoption of the unit prefix micro-, denoted μ, during the creation of the SI in 1960.

In the SI, the systematic name micrometre became the official name of the unit, and μm became the official unit symbol.

Additionally, in American English, the use of "micron" helps differentiate the unit from the micrometer, a measuring device, because the unit's name in mainstream American spelling is a homograph of the device's name. In spoken English, they may be distinguished by pronunciation, as the name of the measuring device is often stressed on the second syllable (/maɪˈkrɒmɪtər/ my-KROM-it-ər), whereas the systematic pronunciation of the unit name, in accordance with the convention for pronouncing SI units in English, places the stress on the first syllable (/ˈmaɪkroʊmiːtər/ MY-kroh-meet-ər).

The plural of micron is normally microns, though micra was occasionally used before 1950.^[9][10][11]

The official symbol for the SI prefix micro- is a Greek lowercase mu.^[12] In Unicode, there is also a micro sign with the code point U+00B5 (µ), distinct from the code point U+03BC (μ) of the Greek letter lowercase mu. According to the Unicode Consortium, the Greek letter character is preferred,^[13] but implementations must recognize the micro sign as well. Most fonts use the same glyph for the two characters.

• Metric prefix
• Metric system
• Orders of magnitude (length)
• Wool measurement