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Comparison between the micro symbol and the Greek letter mu in Linux Libertine

Micro- (Greek letter μ or legacy 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. "mc" is commonly used as a prefix when the character "μ" is not available; for example, "mcg" commonly denotes a microgram. Also the letter u instead of μ is allowed by one of the ISO documents.[2]


Prefix power of 1000 power of 10 Base 10 English word Adoption[nb 1]
Name Symbol Short scale Long scale
yocto y  1000⁻⁸  10⁻²⁴  0.000000000000000000000001  septillionth  quadrillionth 1991
zepto z  1000⁻⁷  10⁻²¹ 0.000000000000000000001000  sextillionth  trilliardth 1991
atto a  1000⁻⁶  10⁻¹⁸ 0.000000000000000001000000  quintillionth  trillionth 1964
femto f  1000⁻⁵  10⁻¹⁵ 0.000000000000001000000000  quadrillionth  billiardth 1964
pico p  1000⁻⁴  10⁻¹² 0.000000000001000000000000  trillionth  billionth 1960
nano n  1000⁻³  10⁻⁹ 0.000000001000000000000000  billionth  milliardth 1960
micro μ  1000⁻²  10⁻⁶ 0.000001000000000000000000  millionth 1873
milli m  1000⁻¹  10⁻³ 0.001000000000000000000000  thousandth 1795
centi c  1000⁻2⁄3   10⁻² 0.010000000000000000000000  hundredth 1795
deci d  1000⁻1⁄3  10⁻¹ 0.100000000000000000000000  tenth 1795
 1000⁰  10⁰ 1.000000000000000000000000  one Unknown
deca da  10001⁄3  10¹ 10.000000000000000000000000  ten 1795
hecto h  10002⁄3  10² 100.000000000000000000000000  hundred 1795
kilo k  1000¹  10³ 1000.000000000000000000000000  thousand 1795
mega M  1000²  10⁶ 1000000.000000000000000000000000  million 1873
giga G  1000³  10⁹ 1000000000.000000000000000000000000  billion  milliard 1960
tera T  1000⁴  10¹² 1000000000000.000000000000000000000000  trillion  billion 1960
peta P  1000⁵  10¹⁵ 1000000000000000.000000000000000000000000  quadrillion  billiard 1975
exa E  1000⁶  10¹⁸ 1000000000000000000.000000000000000000000000  quintillion  trillion 1975
zetta Z  1000⁷  10²¹ 1000000000000000000000.000000000000000000000000  sextillion  trilliard 1991
yotta Y  1000⁸  10²⁴ 1000000000000000000000000.000000000000000000000000  septillion  quadrillion 1991
  1. ^ Prefixes adopted before 1960 already existed before SI. 1873 was the introduction of the CGS system.

Symbol encoding in character sets[edit]

The official symbol for the SI prefix micro- is a Greek lowercase mu (μ).[4] For reasons stemming from its design, 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 1987), at U+00B5 (Alt+0181),[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 (Alt+956). According to The Unicode Consortium, the Greek letter character is preferred,[6] but implementations must recognize the micro sign as well.

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[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 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[edit]


  1. ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), ISBN 92-822-2213-6, archived (PDF) from the original on 2017-08-14
  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 (PDF) (2nd ed.). 1983-05-15. Retrieved 2016-12-14. [1]
  9. ^ Vornorm DIN 66030 [Preliminary standard DIN 66030] (in German). January 1973.
  10. ^ DIN 66030: Darstellungen von Einheitennamen in Systemen mit beschränktem Schriftzeichenvorrat (in German) (1st ed.). 1980.
  11. ^ "Neue Normen für die Informationsverarbeitung". Computerwoche (in German). 1981-01-09. Archived from the original on 2016-12-14. Retrieved 2016-12-14.
  12. ^ DIN 66030:2002-05 - Informationstechnik - Darstellung von Einheitennamen in Systemen mit beschränktem Schriftzeichenvorrat [Information technology - Representation of SI and other units in systems with limited character sets] (in German). Beuth Verlag [de]. May 2002. Retrieved 2016-12-14.
  13. ^ a b c Burtis, Carl A.; Ashwood, Edward R.; Bruns, David E. (2012), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics (5th ed.).
  14. ^ "Commonly Used UCUM Codes for Healthcare Units". HL7 Deutschland e.V. Retrieved 2015-11-21.