Atto denotes a unit one-quintillionth (10^{−18}) the size of the unit named by the stem.

Atto (a) is a unit prefix denoting a variable of 10^{−18} or 0.0000000000000001 in the metric system. The unit multiple was introduced in Resolution 8 at the 12th General Weights and Measures Meeting. It comes from the Danish word atten which means eighteen.

Example:

- Weight of the HIV-1 virus is about 1×10
^{−18}kg which can be written as 1000 ag.