Micro-

This article describes the SI prefix. For other meanings, see Micro (disambiguation).

Micro (μ) is a 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.

Symbol encoding in character sets

For historical reasons, there are two different characters in Unicode, which appear slightly different in some fonts. The micro sign (µ) is encoded in the "Latin-1 Supplement" range identical to ISO/IEC 8859-1 (since 1985), at U+00B5 (Unicode 1.0, 1991). The Greek letter (μ) is encoded in the Greek range at U+03BC.

In circumstances in which only the Latin alphabet is available, the prefix can be represented using the letter u as in um for μm, or uF for μF.^[2] In some contexts, such as pharmacy, microgram is often written "mcg", which is nonstandard.

SI prefixes

Prefix	Symbol	1000m	10n	Decimal	Short scale	Long scale	Since[n 1]
yotta	Y	10008	1024	1000000000000000000000000	Septillion	Quadrillion	1991
zetta	Z	10007	1021	1000000000000000000000	Sextillion	Trilliard	1991
exa	E	10006	1018	1000000000000000000	Quintillion	Trillion	1975
peta	P	10005	1015	1000000000000000	Quadrillion	Billiard	1975
tera	T	10004	1012	1000000000000	Trillion	Billion	1960
giga	G	10003	109	1000000000	Billion	Milliard	1960
mega	M	10002	106	1000000	Million		1960
kilo	k	10001	103	1000	Thousand		1795
hecto	h	10002/3	102	100	Hundred		1795
deca	da	10001/3	101	10	Ten		1795
		10000	100	1	One		–
deci	d	1000−1/3	10−1	0.1	Tenth		1795
centi	c	1000−2/3	10−2	0.01	Hundredth		1795
milli	m	1000−1	10−3	0.001	Thousandth		1795
micro	μ	1000−2	10−6	0.000001	Millionth		1960
nano	n	1000−3	10−9	0.000000001	Billionth	Milliardth	1960
pico	p	1000−4	10−12	0.000000000001	Trillionth	Billionth	1960
femto	f	1000−5	10−15	0.000000000000001	Quadrillionth	Billiardth	1964
atto	a	1000−6	10−18	0.000000000000000001	Quintillionth	Trillionth	1964
zepto	z	1000−7	10−21	0.000000000000000000001	Sextillionth	Trilliardth	1991
yocto	y	1000−8	10−24	0.000000000000000000000001	Septillionth	Quadrillionth	1991
^ The metric system was introduced in 1795 with six prefixes. The other dates relate to recognition by a resolution of the CGPM.

References

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.

See also

• micrometre
• square micrometre
• microsecond