A model equation to determine mean inner porosity of granules ε is proposed. The equation is based on the size analysis of bed particles (agglomeration kinetics) and the measurement of bulk density of feed. Bulk density of a granulated product depends on properties of the raw materials (density, particle size distribution), properties of the wetting liquid, particle concentration in the formed granules (porosity of the granules), as well as on the obtained particle size distribution on which interparticle space volume depends. In order to verify the proposed model, the process of drum granulation was tested for the following range of changing parameters: drum diameter D = 0.25 to 0.4 m, filling of the drum with granular material k = 5 to 20% of the inner volume, relative rotational speed of the drum nw= 0.15 to 0.375 of critical velocity. The granular bed was wetted drop-wise during the drum rotations, at constant liquid supply rate Q =1 cm³/s. During the process, at constant time intervals, samples were taken from the drum and on their basis values of bulk density of the feed as well as its particle size distribution, and inner porosity of the granules for a given size fraction were determined. The volume of liquid accumulated in the inner pores of the granules was specified basing on the mass balance before and after drying. Results of the research confirmed validity of the proposed model.