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.[2] 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 The letter u, instead of μ, was allowed by an ISO document,[3] but that document has been withdrawn in 2001, however DIN 66030:2002 still allows this substitution.[4]


Prefix Base 10 Decimal Adoption
[nb 1]
Name Symbol
quetta Q 1030 1000000000000000000000000000000 2022[6]
ronna R 1027 1000000000000000000000000000 2022
yotta Y 1024 1000000000000000000000000 1991
zetta Z 1021 1000000000000000000000 1991
exa E 1018 1000000000000000000 1975
peta P 1015 1000000000000000 1975
tera T 1012 1000000000000 1960
giga G 109 1000000000 1960
mega M 106 1000000 1873
kilo k 103 1000 1795
hecto h 102 100 1795
deca da 101 10 1795
100 1
deci d 10−1 0.1 1795
centi c 10−2 0.01 1795
milli m 10−3 0.001 1795
micro μ 10−6 0.000001 1873
nano n 10−9 0.000000001 1960
pico p 10−12 0.000000000001 1960
femto f 10−15 0.000000000000001 1964
atto a 10−18 0.000000000000000001 1964
zepto z 10−21 0.000000000000000000001 1991
yocto y 10−24 0.000000000000000000000001 1991
ronto r 10−27 0.000000000000000000000000001 2022
quecto q 10−30 0.000000000000000000000000000001 2022
  1. ^ Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873.

Symbol encoding in character sets

The official symbol for the SI prefix micro is a Greek lowercase mu (μ).[7] For reasons stemming from its design, Unicode has two different character codes for the letter, with slightly different appearance 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),[8] 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,[9] 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,[10] 1983[11]), DIN 66030 (Vornorm 1973;[12] 1980,[13][14] 2002[4]) 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, or in the common abbreviation UC for microcontroller (µC). 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.[15] The two alternatives are to abbreviate as "mcg"[15][2] 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[15]), 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.[16]

See also


  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 2021-06-04, retrieved 2021-12-16
  2. ^ a b "ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations: Abbreviations for Doses/Measurement Units". Recommendations. Institute for Safe Medication Practices (ISMP). 2021-02-05. Archived from the original on 2022-12-24. Retrieved 2022-12-24. Error-Prone Abbreviations, Symbols, and Dose Designations: µg […] Intended Meaning: Microgram […] Misinterpretation: Mistaken as mg […] Best Practice: Use mcg […]
  3. ^ ISO 2955, table 2.
  4. ^ a b 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.
  5. ^ Biology by Campbell & Reece, tenth edition. Ch. 6 "A Tour of the Cell". p. 98.
  6. ^ "On the extension of the range of SI prefixes". 2022-11-18. Retrieved 2023-02-05.
  7. ^ Prefixes of the International System of Units, International Bureau of Weights and Measures (page visited on 9 May 2016).
  8. ^ (Unicode 1.0, 1991)
  9. ^ Unicode Technical Report #25
  10. ^ ISO 2955-1974: lnformation processing - Representations of SI and other units for use in systems with limited character sets (1st ed.). 1974.
  11. ^ "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]
  12. ^ Vornorm DIN 66030 [Preliminary standard DIN 66030] (in German). January 1973.
  13. ^ DIN 66030: Darstellungen von Einheitennamen in Systemen mit beschränktem Schriftzeichenvorrat (in German) (1st ed.). 1980.
  14. ^ "Neue Normen für die Informationsverarbeitung". Computerwoche (in German). 1981-01-09. Archived from the original on 2016-12-14. Retrieved 2016-12-14.
  15. ^ a b c Burtis, Carl A.; Ashwood, Edward R.; Bruns, David E. (2012), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics (5th ed.), ISBN 978-1455759422.
  16. ^ "Commonly Used UCUM Codes for Healthcare Units". HL7 Deutschland e.V. 2015-11-21. Archived from the original on 2022-10-06. Retrieved 2022-12-24.