Atto-

Atto (symbol a) is a unit prefix in the metric system denoting a factor of 10−18 or 0.000000000000000001.

The unit multiple was adopted at the 12th General Conference on Weights and Measures (CGPM) in Resolution 8.[1] It is derived from the Danish word atten, meaning "eighteen".

Examples of its use:

* Weight of the HIV-1 virus is about 1×10−18 kg or 1×10−15 g, which can be written as 1 fg or 1000 ag.
* More examples available.

SI prefixesv * d * e

Prefix  | Base 10  | Decimal | English word  | Adoption[nb 1]

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|
|
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Name  | Symbol  | Short scale | Long scale
yotta | Y  |  1024 | 1000000000000000000000000 |  septillion  |  quadrillion  | 1991
zetta | Z  |  1021 | 1000000000000000000000 |  sextillion  |  trilliard  | 1991
exa | E  |  1018 | 1000000000000000000 |  quintillion  |  trillion  | 1975
peta | P  |  1015 | 1000000000000000 |  quadrillion  |  billiard  | 1975
tera | T  |  1012 | 1000000000000 |  trillion  |  billion  | 1960
giga | G  |  109 | 1000000000 |  billion  |  milliard  | 1960
mega | M  |  106 | 1000000 |  million  | 1873
kilo | k  |  103 | 1000 |  thousand  | 1795
hecto | h  |  102 | 100  |  hundred  | 1795
deca | da  |  101 | 10  |  ten  | 1795
|  100 | 1  |  one  | –
deci | d  |  10−1 | 0.1  |  tenth  | 1795
centi | c  |  10−2 | 0.01  |  hundredth  | 1795
milli | m  |  10−3 | 0.001  |  thousandth  | 1795
micro | μ  |  10−6 | 0.000001 |  millionth  | 1873
nano | n  |  10−9 | 0.000000001 |  billionth  |  milliardth  | 1960
pico | p  |  10−12 | 0.000000000001 |  trillionth  |  billionth  | 1960
femto | f  |  10−15 | 0.000000000000001 |  quadrillionth  |  billiardth  | 1964
atto | a  |  10−18 | 0.000000000000000001 |  quintillionth  |  trillionth  | 1964
zepto | z  |  10−21 | 0.000000000000000000001 |  sextillionth  |  trilliardth  | 1991
yocto | y  |  10−24 | 0.000000000000000000000001 |  septillionth  |  quadrillionth  | 1991

1. ↑ Prefixes adopted before 1960 already existed before SI. 1873 was the introduction of the CGS system.

## 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

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