Rapid filtration as one of the important unit processes in water treatment requires a properly designed process of rinsing filter beds. An important issue affecting the correct backwash hydraulics is determining the minimum fluidization velocity. This velocity depends on the physical characteristics of filter materials, such as density, porosity, grain size and the shape of bed grains. Determining these values makes it possible to calculate the minimum fluidization speed, which is important for determining the moment of the beginning of the bed expansion and determining the minimum flow for which the rinsing pump is selected. As part of the research, the physical parameters of the most commonly used filter materials, such as sand and hydranthracite, were determined, as well as the high- and low-density expanded clay deposits that have been increasingly used in Europe in recent years. The measured values were used in the calculation of the minimum fluidization velocity (Vmf) using the equations most commonly used in the literature. The results of calculations of the minimum fluidization velocities of the beds were compared with the results of experimental measurements carried out on a semi-technical laboratory scale. Analyzing the obtained results, it was found that the minimum fluidization velocity for quartz sand and high-density expanded clay is best determined using Gidaspow's formula, while the minimum fluidization velocity for hyroanthracite and low-density expanded clay is most precisely calculated by the formula used by Majcherek.