Micro-

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".

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

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]

Examples
SI prefixes
PrefixBase 10 DecimalEnglish word Adoption[nb 1]Etymology
NameSymbolShort scaleLong scaleLanguageSource word
yottaY1024 1000000000000000000000000septillionquadrillion1991Latin eight[nb 2]
zettaZ1021 1000000000000000000000sextilliontrilliard1991Latinseven[nb 2]
exaE1018 1000000000000000000quintilliontrillion1975Greeksix
petaP1015 1000000000000000quadrillionbilliard1975Greekfive[nb 2]
teraT1012 1000000000000trillionbillion1960Greekfour,[nb 2] monster
gigaG109 1000000000billionmilliard1960Greekgiant
megaM106 1000000million1873Greekgreat
kilok103 1000thousand1795Greekthousand
hectoh102100 hundred1795Greekhundred
decada10110 ten1795Greekten
1001one–
decid10−1 0.1tenth1795Latinten
centic10−2 0.01hundredth1795Latinhundred
millim10−3 0.001thousandth1795Latinthousand
microμ10−6 0.000001millionth1873Greeksmall
nanon10−9 0.000000001billionthmilliardth1960Greekdwarf
picop10−12 0.000000000001trillionthbillionth1960Spanishpeak, a little bit
femtof10−15 0.000000000000001quadrillionthbilliardth1964Danish fifteen, Fermi[nb 3]
attoa10−18 0.000000000000000001quintillionthtrillionth1964Danisheighteen
zeptoz10−21 0.000000000000000000001sextillionthtrilliardth1991Latinseven[nb 2]
yoctoy10−24 0.000000000000000000000001septillionthquadrillionth1991Latineight[nb 2]
  1. Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873.
  2. Part of the beginning of the prefix was modified from the word it was derived from, ex: "peta" (prefix) vs "penta" (source word).
  3. The fermi was introduced earlier with the symbol "fm", which prompted the reinterpretation of the "f" as a prefix to "m", with femto being derived from the Danish word femten due to its similarity.

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 (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. 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

References
  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. 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.
  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. May 2002. Retrieved 2016-12-14.
  13. Burtis, Carl A.; Ashwood, Edward R.; Bruns, David E. (2012), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics (5th ed.), ISBN 978-1455759422.
  14. "Commonly Used UCUM Codes for Healthcare Units". HL7 Deutschland e.V. Retrieved 2015-11-21.
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