Theoretical particle number and particle size were calculated by appropriate handling of the Nielsen-model from experimental induction period and supersaturation ratio in three crystallization systems. Also the particle size distribution of the final product was measured. The maximum difference between theoretical and experimental particle sizes was smaller than one order of magnitude in all the cases which indicates that this model gives real information about the investigated systems. The method is applicable also for organic systems and cooling crystallization and moreover can predict the possible changes in the particle size.