# Micrometre

The micrometre (Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, 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).
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, and for grading wool by the diameter of the fibres. The width of a single human hair ranges from approximately 20 to 200 μm.

## Examples

Between 1 μm and 10 μm:
* 1–10 μm – length of a typical bacterium
* 3–8 μm – width of strand of spider web silk
* 5 μm – length of a typical human spermatozoon's head
* 6–8 μm – diameter of a typical red blood cell
* 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
* 17 to 181 μm – diameter of human hair
* 70 to 180 μm – thickness of paper

## SI standardization

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. 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.
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.

## Symbol

The official symbol for the SI prefix micro- is a Greek lowercase mu. Unicode has inherited U+00B5 µ MICRO SIGN from ISO/IEC 8859-1, distinct from the code point U+03BC μ GREEK SMALL LETTER MU. According to the Unicode Consortium, the Greek letter character is preferred, but implementations must recognize the micro sign as well for compatibility with legacy character sets. Most fonts use the same glyph for the two characters.
Before desktop publishing became commonplace, it was customary to render the symbol μ in texts produced with mechanical typewriters by combining a slightly lowered slash with the letter u. For example, "15 μm" would appear as "15$_/$um". This gave rise in early word processing to substituting just the letter u for the symbol if the Greek letter μ was not available, as in "15 um".
The Unicode CJK Compatibility block contains square forms of some Japanese katakana measure and currency units. U+3348 ㍈ SQUARE MIKURON corresponds to ミクロン mikuron.