Femto-

Femto- (symbol f) is a unit prefix in the metric system denoting a factor of 10−15 or 0.000 000  000  000  001. Adopted by the 11th General Conference on Weights and Measures, it was added in 1964 to the SI. It is derived from the Danish word femten, meaning "fifteen".

Examples of use: The femtometre shares the unit symbol (fm) with the older non-SI unit fermi, to which it is equivalent. The fermi, named in honour of Enrico Fermi, is often encountered in nuclear physics.
 * Weight of the HIV-1 virus is about 1 x 10−18 kg, which equals 1 x 10−15 g or 1 fg.
 * a proton has a diameter of about 1.6 to 1.7 femtometres.
 * More examples available.