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. 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 prefixes

Prefix		Base 10	Decimal	English word		Adoption
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