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Micro (Greek letter μ (U+03BC) or the legacy symbol µ (U+00B5)) 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 is the Greek letter μ (mu). It is the only SI prefix which uses a character not from the Latin alphabet. "mc" is commonly used as a prefix when the character "μ" is not available; for example, "mcg" commonly denotes a microgram. This may be ambiguous in rare circumstances in that mcg could also be read as a micrigram, i.e. 10−14 g; however the prefix micri is not standard, nor widely known, and is considered obsolete. The letter u, instead of μ, is allowed by an ISO document.[2]


Prefix Base 10 Decimal English word Adoption[nb 1]
Name Symbol Short scale Long scale
yotta Y  1024 1000000000000000000000000  septillion  quadrillion 1991
zetta Z  1021 1000000000000000000000  sextillion  trilliard 1991
exa E  1018 1000000000000000000  quintillion  trillion 1975
peta P  1015 1000000000000000  quadrillion  billiard 1975
tera T  1012 1000000000000  trillion  billion 1960
giga G  109 1000000000  billion  milliard 1960
mega M  106 1000000  million 1873
kilo k  103 1000  thousand 1795
hecto h  102 100  hundred 1795
deca da  101 10  ten 1795
 100 1  one
deci d  10−1 0.1  tenth 1795
centi c  10−2 0.01  hundredth 1795
milli m  10−3 0.001  thousandth 1795
micro μ  10−6 0.000001  millionth 1873
nano n  10−9 0.000000001  billionth  milliardth 1960
pico p  10−12 0.000000000001  trillionth  billionth 1960
femto f  10−15 0.000000000000001  quadrillionth  billiardth 1964
atto a  10−18 0.000000000000000001  quintillionth  trillionth 1964
zepto z  10−21 0.000000000000000000001  sextillionth  trilliardth 1991
yocto y  10−24  0.000000000000000000000001  septillionth  quadrillionth 1991
  1. Prefixes adopted before 1960 already existed before SI. 1873 was the introduction of the CGS system.

Symbol encoding in character sets

The official symbol for the SI prefix micro is a Greek lowercase mu (μ).[4] For reasons stemming from its design, Unicode has two different character codes for the letter, 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 1987), at U+00B5 (),[5] residing at this code point also in DEC MCS (since 1983) and ECMA-94 (since 1985). The Greek letter (μ) is encoded in the Greek range at U+03BC (). According to The Unicode Consortium, the Greek letter character is preferred,[6] but implementations must recognize the micro sign as well. This distinction also occurs in some legacy code pages, notably Windows-1253.

In circumstances in which only the Latin alphabet is available, ISO 2955 (1974,[7] 1983[8]), DIN 66030 (Vornorm 1973;[9] 1980,[10][11] 2002[12]) and BS 6430 (1983) allow the prefix μ to be substituted by the letter u (or even U, if lowercase letters are not available), as, for example, in um for μm, or uF for μF. Similar, capacitor values according to the RKM code defined in IEC 60062 (IEC 62) (since 1952), EN 60062, DIN 40825 (1973), BS 1852 (1974), IS 8186 (1976) etc. can be written as 4u7 (or 4U7) instead of 4μ7 if the Greek letter μ is not available.

Other abbreviating conventions

In some health care institutions, house rules deprecate the standard symbol for microgram, "μg", in prescribing or chart recording, because of the risk of giving an incorrect dose because of the misreading of poor handwriting.[13] The two alternatives are to abbreviate as "mcg"[13] or to write out "microgram" in full (see also List of abbreviations used in medical prescriptions). But this deprecation, focused on avoiding incorrect dosing 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 do not[13]), and in physical sciences academia, "μg" remains the sole official abbreviation.

In medical data exchange according to the Health Level 7 (HL7) standard, the μ can be replaced by u as well.[14]

See also


  1. International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), ISBN 92-822-2213-6, http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf 
  2. ISO 2955, table 2.
  3. Biology by Campbell & Reece, tenth edition. Ch. 6 "A Tour of the Cell". p. 98.
  4. Prefixes of the International System of Units, International Bureau of Weights and Measures (page visited on 9 May 2016).
  5. (Unicode 1.0, 1991)
  6. Unicode Technical Report #25
  7. ISO 2955-1974: lnformation processing - Representations of SI and other units for use in systems with limited character sets (1st ed.). 1974. 
  8. "Table 2". ISO 2955-1983: lnformation processing - Representations of SI and other units for use in systems with limited character sets (2nd ed.). 1983-05-15. http://isotc.iso.org/livelink/livelink/4289384/ISO_2955-1983E_repr_of_SI_units_with_limited_char_sets.pdf?func=doc.Fetch&nodeid=4289384. Retrieved 2016-12-14.  [1]
  9. (in de) Vornorm DIN 66030. January 1973. 
  10. (in de) DIN 66030: Darstellungen von Einheitennamen in Systemen mit beschränktem Schriftzeichenvorrat (1st ed.). 1980. 
  11. "Neue Normen für die Informationsverarbeitung" (in de). Computerwoche. 1981-01-09. http://www.computerwoche.de/a/neue-normen-fuer-die-informationsverarbeitung,1184901. Retrieved 2016-12-14. 
  12. (in de) DIN 66030:2002-05 - Informationstechnik - Darstellung von Einheitennamen in Systemen mit beschränktem Schriftzeichenvorrat. Beuth Verlag (de). May 2002. https://www.beuth.de/de/norm/din-66030/50570368. Retrieved 2016-12-14. 
  13. 13.0 13.1 13.2 Burtis, Carl A.; Ashwood, Edward R.; Bruns, David E. (2012), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics (5th ed.), ISBN 978-1455759422, https://books.google.com/books?id=BBLRUI4aHhkC&pg=PT2241. 
  14. "Commonly Used UCUM Codes for Healthcare Units". HL7 Deutschland e.V.. http://download.hl7.de/documents/ucum/ucumdata.html.