Percent crystallinity reports the fractional amount of material that has crystallized, with the remainder being amorphous (unstructured in three dimensions). The percent crystallinity of a material can be determined even if the crystalline phases are unknown.

The determination of crystallinity is limited by the crystallite size. Very small crystallite sizes, particularly for unknown materials, can increase the uncertainty of the result. Percent crystallinity is performed by deconvolution of the amorphous and crystalline peaks in the diffraction pattern. While there may be many phases in a compound and one of them may be responsible for most of the amorphous material, only one value for crystallinity is reported for the entire sample.