The advantage of the theoretical description of a particle size distribution as a continuous differentiable function is its simple formulation without losing mathematical correctness. Real particle size analysis data are always obtained from finite quantities and by using measuring technique with a limited number of particle size classes, so a particle size distribution determined by a particle size measurement is always a discrete distribution and is therefore not differentiable. Moreover, practical representation and evaluation of real particle size analysis data is often based on certain assumptions. In further evaluations, calculation practices are not always in strict accordance with prior assumptions. In this article common practices in the calculation of any density distribution by using the concept of moments are discussed and differences in the results are pointed out.