Crystallite size is reported in nm for each individual crystallographic phase. The crystallite size is the average size of a coherent scattering domain (perfect arrangement of unit cells or perfect crystal). The crystallite size may affect dissolution, material strength, catalytic activity or product aging. Note that crystallite size and grain size are not necessarily the same, since a grain may be composed of multiple crystallites

Determination of crystallite size by XRD is ideal when crystallites are too small for optical methods. In addition, the number of crystallites illuminated by XRD is typically greater than 106.

If the crystallite size is asymmetric (crystallites have preferred growth directions similar to graphite), two crystallite size components can be refined (Lc and Lg).

The principle of crystallite size measurements by x-ray diffraction is based upon the fact that the Fourier transform of different size crystallites varies inversely with the size of the crystallite. X-rays in the 0.15 nm range can measure crystallite sizes from approximately 2nm to 500nm.

The traditional measure of crystallite size is by the Scherrer equation, which shows that the crystallite size is inversely proportional to the peak width (FWHM), using the equation

Crystallite size (nm) = kl/ FWHM (cos 2q)

Note that one average crystallite size is reported for the entire sample and the crystallite size distribution can be estimated in some circumstances.