Method FW1/4/5M allows for quick Grain Size Distribution (GSD) calculation. You need to specify two peak widths: measured at 1/5 and 4/5 of maximum of the diffraction peak (FW1/5M and FW4/5M, respectively). The metod takes into account only size-broadening of the diffraction maxima.

Link: FW1/5,4/5M method for evaluation of Grain Size Distribution – on-line calculator