The units of the structure-factor amplitude depend on the incident radiation. For X-ray crystallography they are multiples of the unit of scattering by a single electron (2.82 m); for neutron scattering by atomic nuclei the unit of scattering length of  m is commonly used.
The above discussion uses the wave vectors  and . However, crystallography  often uses wave vectors  and . Therefore, when comparing equations from different sources, the factor  may appear and disappear, and care to maintain consistent quantities is required to get correct numerical results.