Femtometer

The femtometer, also called fermi, after the physicist Enrico Fermi, is the unit of length equal to one billionth (1/1,000,000,000,000,000) of a meter. It is abbreviated fm.

One fermi is equal to 1×10⁻¹⁵ m, and one hundred thousandth of an angstrom (Å), which is a distance close to the radius of an atom (1.25 Å for aluminum).

It is used in measurements of the atomic nucleus. Neutrons and protons are about 0.833 ± 0.010 fm in radius. Electrons do not show size below 10⁻³ fermis resolutions (10⁻⁸ Å).