Femtometre ($1\mathrm{fm}=10^{-15}\mathrm{m}$)

A femtometre is $10^{-15}$ metre, written $\mathrm{fm}$. It is a convenient unit of measurement for the dimensions of an atomic nucleus, rather than the much larger dimensions of a chemical atom. For example, a length of $5\mathrm{fm}$ is $5\times 10^{-15}\mathrm{m}$.

Scattering probes such small lengths through momentum transfer rather than a miniature mechanical ruler. The relation between wavelength and momentum explains why increasingly short-distance structure requires increasingly large momentum transfer.