A series of smectites was investigated to reveal the thickness distribution of crystallites. The Fourier decomposition technique of Bertaut-Warren-Averbach (MudMaster program) was applied to XRD reflections of <0.2 μm, glycolated Na-clays. It was shown that the thickness distribution is lognormal and the mean thickness ranges from 5.7 to 12.3 nm (3.4–7.3 layers). At higher humidities, characteristic of TEM sample preparation, the mean thicknesses decrease, but the differences in mean thickness between samples are preserved. Beidellites have the thickest crystallites. The relationships between the mean thickness, volume-weighted mean thickness and the parameters of the lognormal distribution were established. Calculation of these parameters is possible, using the area and the maximum intensity of the 001 reflection. The influence of the fluctuations of d-spacing on the peak width was shown. Smectite crystallites, dispersed into individual layers during infinite osmotic swelling and rebuilt through coagulation, recover their original mean thickness and thickness distribution.