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This article is about the SI prefix. For other uses, see Micro (disambiguation).

Micro (symbol µ) is a unit prefix in the metric system denoting a factor of 10−6 (one millionth).[1] Confirmed in 1960, the prefix comes from the Greek μικρός (mikrós), meaning "small".

The symbol for the prefix comes from the Greek letter μ (mu). It is the only SI prefix which uses a character not from the Latin alphabet.


Prefix 1000m 10n Decimal English word Since[n 1]
name symbol short scale long scale
yotta Y  10008  1024 1000000000000000000000000  septillion  quadrillion 1991
zetta Z  10007  1021 1000000000000000000000  sextillion  thousand trillion 1991
exa E  10006  1018 1000000000000000000  quintillion  trillion 1975
peta P  10005  1015 1000000000000000  quadrillion  thousand billion 1975
tera T  10004  1012 1000000000000  trillion  billion 1960
giga G  10003  109 1000000000  billion  thousand million 1960
mega M  10002  106 1000000             million 1960
kilo k  10001  103 1000             thousand 1795
hecto h  10002/3  102 100             hundred 1795
deca da  10001/3  101 10             ten 1795
 10000  100 1             one
deci d  1000−1/3  10−1 0.1             tenth 1795
centi c  1000−2/3   10−2 0.01             hundredth 1795
milli m  1000−1  10−3 0.001             thousandth 1795
micro μ  1000−2  10−6 0.000001             millionth 1960
nano n  1000−3  10−9 0.000000001  billionth  thousand millionth 1960
pico p  1000−4  10−12 0.000000000001  trillionth  billionth 1960
femto f  1000−5  10−15 0.000000000000001  quadrillionth  thousand billionth 1964
atto a  1000−6  10−18 0.000000000000000001  quintillionth  trillionth 1964
zepto z  1000−7  10−21 0.000000000000000000001  sextillionth  thousand trillionth 1991
yocto y  1000−8  10−24  0.000000000000000000000001  septillionth  quadrillionth  1991
  1. ^ The metric system was introduced in 1795 with six metric prefixes. The other dates relate to recognition by a resolution of the General Conference on Weights and Measures (CGPM).

Symbol encoding in character sets[edit]

For historical reasons, there are two different characters in Unicode, which appear slightly different in some fonts, although most fonts use the same glyph. The micro sign (µ) is encoded in the "Latin-1 Supplement" range identical to ISO/IEC 8859-1 (since 1985), at U+00B5 (Unicode 1.0, 1991). The Greek letter (μ) is encoded in the Greek range at U+03BC. According to The Unicode Consortium, the Greek letter character is preferred,[4] but implementations must recognize the micro sign as well.

In circumstances in which only the Latin alphabet is available, the prefix can (unofficially) be represented using the letter u as in um for µm, or uF for µF.[5]

Other abbreviating conventions[edit]

In some health care institutions, house rules deprecate the standard symbol for microgram, "µg", in prescribing or chart recording, because of the risk of misdose via the misreading of poor handwriting.[6] The two alternatives are to abbreviate as "mcg"[6] or to write out "microgram" in full (see also List of abbreviations used in medical prescriptions). But this deprecation, focused on bedside misdose avoidance in contexts where handwriting is often present, does not extend to all health-care contexts and institutions (for example, some clinical laboratories' reports adhere to it, whereas others don't[6]), and in physical sciences academia, "µg" remains the sole official abbreviation.

See also[edit]


  1. ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), ISBN 92-822-2213-6 
  2. ^ http://www.nikon.com/about/feelnikon/universcale/index_f.htm
  3. ^ Biology by Campbell & Reece tenth edition page 98 Ch. 6 A Tour of the Cell
  4. ^ Unicode Technical Report #25
  5. ^ ISO 2955, Table 2.
  6. ^ a b c Burtis, Carl A.; Ashwood, Edward R.; Bruns, David E. (2012), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics (5th ed.).