Atto-

This redirect 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] Error: {{Lang}}: text has italic markup (help), 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	Adoption
Name	Symbol
quetta	Q	1030	1000000000000000000000000000000	2022
ronna	R	1027	1000000000000000000000000000
yotta	Y	1024	1000000000000000000000000	1991
zetta	Z	1021	1000000000000000000000
exa	E	1018	1000000000000000000	1975
peta	P	1015	1000000000000000
tera	T	1012	1000000000000	1960
giga	G	109	1000000000
mega	M	106	1000000	1873
kilo	k	103	1000	1795
hecto	h	102	100
deca	da	101	10
—	—	100	1	—
deci	d	10−1	0.1	1795
centi	c	10−2	0.01
milli	m	10−3	0.001
micro	μ	10−6	0.000001	1873
nano	n	10−9	0.000000001	1960
pico	p	10−12	0.000000000001
femto	f	10−15	0.000000000000001	1964
atto	a	10−18	0.000000000000000001
zepto	z	10−21	0.000000000000000000001	1991
yocto	y	10−24	0.000000000000000000000001
ronto	r	10−27	0.000000000000000000000000001	2022
quecto	q	10−30	0.000000000000000000000000000001
Notes