|1.0000×10−6 m||1.00000 μm|
|US customary units (Imperial units)|
|3.2808×10−6 ft||39.370×10−6 in|
|Look up micrometre in Wiktionary, the free dictionary.|
The micrometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: µm) or micrometer (American spelling), also commonly known as a micron, is an SI derived unit of length equaling 1×10−6 of a 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 0.000039 inch). The symbol µm is sometimes rendered as um if the symbol µ cannot be used, or if the writer is not aware of the distinction.
The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of cells and bacteria and is also commonly used in plastics manufacturing. Micrometres are the standard for grading wool (referring to the diameter of wool fibres). Any wool finer than 25 µm can be used for garments, while coarser grades are used for outerwear, rugs and carpets. A human hair is about 90 µm wide.
The symbol for the SI prefix micro- is a Greek lowercase mu. In Unicode, it has the codepoint U+00B5 (µ), distinct from the codepoint U+03BC (μ) of the Greek letter lowercase mu, so that machines can recognize it as the SI prefix symbol rather than as a letter. Most fonts use the same glyph for the two characters.
The term micron and the symbol µ were officially accepted for use in isolation to denote the micrometre, between 1879 and 1967, but officially revoked by the International System of Units (SI) in 1967. This was necessary because that older usage became 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.
Nevertheless, in practice, "micron" remains a widely used term in preference to "micrometre" in many English-speaking countries, both in academic science (including geology, biology, physics, and astronomy) and in applied science and industry (including machining, the semiconductor industry, and plastics manufacturing). 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 despite being homographs they are not homophones; the name of the measuring device is invariably stressed on the second syllable, 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.)
This older usage was pluralized either as "microns" or "micra", the latter being arguably more correct, although that debate became less relevant after 1967, as the micron and its plural forms have been deprecated for almost half a century.