atto-

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

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. It is derived from the Danish word atten, meaning "eighteen".

Examples of its use:

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

SI prefixes

Prefix 1000m 10n Decimal English word Adoption Name Symbol Short scale Long scale yotta Y  10008  1024 1000000000000000000000000  septillion  quadrillion 1991 zetta Z  10007  1021 1000000000000000000000  sextillion  thousand trillion or trilliard 1991 exa E  10006  1018 1000000000000000000  quintillion  trillion 1975 peta P  10005  1015 1000000000000000  quadrillion  thousand billion or billiard 1975 tera T  10004  1012 1000000000000  trillion  billion 1960 giga G  10003  109 1000000000  billion  thousand million or milliard 1960 mega M  10002  106 1000000             million 1960 (1873) kilo k  10001  103 1000             thousand 1960 (1795) hecto h  10002/3  102 100             hundred 1960 (1795) deca da  10001/3  101 10             ten 1960 (1795)  10000  100 1             one – deci d  1000−1/3  10−1 0.1             tenth 1960 (1795) centi c  1000−2/3   10−2 0.01             hundredth 1960 (1795) milli m  1000−1  10−3 0.001             thousandth 1960 (1795) micro μ  1000−2  10−6 0.000001             millionth 1960 (1873) nano n  1000−3  10−9 0.000000001  billionth  thousand millionth 1960 pico p  1000−4  10−12 0.000000000001  trillionth  billionth 1960 femto f  1000−5  10−15 0.000000000000001  quadrillionth  thousand billionth 1964 atto a  1000−6  10−18 0.000000000000000001  quintillionth  trillionth 1964 zepto z  1000−7  10−21 0.000000000000000000001  sextillionth  thousand trillionth 1991 yocto y  1000−8  10−24  0.000000000000000000000001  septillionth  quadrillionth  1991