# Micrometre

1 micrometre =
SI units
1.0000×10−6 m 1.00000 μm
US customary / Imperial units
3.2808×10−6 ft 39.370×10−6 in

A micrometre or micrometer (pron.: /ˈmkrmtər/, symbol µm) 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 inches). The symbol µm is sometimes rendered as um if the symbol µ cannot be used.

The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of cells and bacteria.[1]

The symbol for the SI prefix micro-, µ, is a Greek lowercase mu, μ. In Unicode, it has a codepoint distinct from that of the Greek letter lowercase mu so that it can be recognized as a symbol and not a letter. Many fonts use the same glyph for the two characters.

## ISI Standardisation

The term micron and the symbol µ, representing the micrometre, were officially accepted between 1879 and 1967, but officially revoked by the ISI in 1967.[2]

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).[citation needed]

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. The term "micron" is still extensively used in most English-speaking countries.[citation needed]

In engineering it is common to use the micron as a replacement for the traditional thou or mil, each of which represent a thousandth of an inch. 1 thou = 1 mil = 25.4 microns. So a trash bag (in British English: a rubbish bag) may be originally specified as 0.35 mil thick, but stated as 8.89 microns. (0.35 mils or 8.89 µm)