Femtometre (in American English femtometer) is an SI measure of length that is equal to 10-15(femto-) of a metre. Its abbreviation is fm and 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 2.5 fm in diameter. In the parlance of particle physicists, a femtometre is often called a 'fermi' (same abbreviation), after the physicist Enrico Fermi.