The purpose of this work is to calculate the process of separation of particles of loose material in a layer under the action of vertical or spatial vibrations. This effect still does not have a satisfactory theoretical explanation. Well-known works, which consider the qualitative characteristics of "rising" or "sinking" of particles, have a phenomenological approach. There is also a probabilistic approach to this problem without taking into account the characteristics of particles and vibration parameters. In this work, bulk material is considered as having particles of different sizes and densities. As a hypothesis, it is assumed that the particles have a spherical shape, the amplitude of harmonic low-frequency vibrations of the surface is proportional to the size of the main mass of the particles, at the moment of separation from the surface, all particles are given the same speed, but a different amount of energy, which is proportional to their mass. The friction coefficient between the particles is also taken into account. According to the calculation of the trajectory of the particles and their position at the end of the separation movement, the trajectories of the main mass of particles are compared with those that differ in size or density. Calculation of the trajectories for a series of detachment movements of the studied particles allows to determine the speed of the separation process, that is, the speed of their advancement in the material layer. It is shown that the speed of distribution of particles depends on the difference in size, density of individual particles, frequency and amplitude of vertical oscillations.