Femtometre

The femtometre is a unit used to measure length in the metric system, represented by the symbol fm. It is equal to one quadrillionth of a meter: 10-15 m, or 1 m / 1,000,000,000,000,000. It is also called a fermi, named after Enrico Fermi. 1 femtometre is equal to 0.001 picometres or 1000 attometres.
1 femtometre is similar in size to a proton or neutron.