In the
metric system, a microgram or microgramme is a
unit of
mass equal to one millionth () of a
gram
The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram.
Originally defined as of 1795 as "the absolute weight of a volume of pure water equal to th ...
. The unit symbol is μg according to the
International System of Units (SI); the recommended symbol in the United States and United Kingdom when communicating medical information is mcg. In μg the prefix symbol for
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). Confirmed in 1960, the prefix comes from the Greek ('), meaning "small".
The symbol for th ...
is the Greek letter
μ (mu).
Abbreviation and symbol confusion
When the Greek lowercase "μ" (mu) in the symbol μg is typographically unavailable, it is occasionally – although not properly – replaced by the Latin lowercase "u".
The United States-based
Institute for Safe Medication Practices (ISMP) and the U.S.
Food and Drug Administration (FDA) recommend that the symbol μg should not be used when communicating medical information due to the risk that the prefix
μ (micro-) might be misread as the prefix
m (milli-), resulting in a thousandfold overdose. The ISMP recommends the non-
SI symbol mcg instead. However, the abbreviation mcg is also the symbol for an obsolete
centimetre–gram–second system of units
The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units a ...
unit of measure known as millicentigram, which is equal to 10 μg.
Gamma (symbol:
γ) is a deprecated non-SI unit of
mass equal to 1 μg.
NIST Handbook 133 - 2018Appendix E. General Tables of Units of Measurement, page 159 (17)
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A fullwidth version of the "microgram" symbol is encoded by Unicode at code point for use in CJK contexts.
See also
* List of SI prefixes
* Orders of magnitude (mass), listing a few items that have a mass of around 1 μg.
References
{{reflist
SI derived units
Units of mass