Femtometre (American spelling: femtometer, symbol:fm) is an SI measure of length that is equal to 10⁻¹⁵ (femto) of a metre. It is commonly used in measuring the diameter of atomic nuclei. The diameter of an atomic nucleus is up to about 15 fm. Neutrons and protons are about 1 fm in diameter. In the parlance of particle physicists, a femtometre is often called a fermi (same symbol), after the physicist Enrico Fermi.

• 1 fm = 1 × 10⁻¹⁵ metre
• 1000 fm = 1 picometre

attometre << femtometre << picometre

See also

• 1 E-15 m
• SI
• SI prefix
• Orders of magnitude