Bellies derived from the commercial population of pig carcasses are diverse in terms of tissue composition. Knowledge of the factors influencing it and the expected results, permits quick and easy evaluation of raw material. The study was designed to determine the factors affecting the tissues composition of pork bellies and to estimate their lean meat content. The research population (n=140 pig carcasses) was divided into groups according to sex (gilts, barrows), half-carcass mass (<40, 40 to 43.9, 44 to 46.9, ⩾47 kg) and lean meat content class: S (⩾60%), E (55% to 60%), U (50% to 55%), R (<50%). Bellies were subjected to a detailed dissection. Half-carcass mass affected the levels of all the analysed parameters. The only exception was the mass of the fat with the skin in the 40 to 43.9 kg group, for which the value did not differ statistically between the two groups <40 and 44 to 46.9 kg. Decrease in lean meat content affected the growth of the fat and skin mass in a linear way. No differences were observed between class S and E in terms of belly muscle mass. A 0.37% higher share of belly in the half-carcass was found for barrows (P<0.001), although bellies issued from barrows were characterized by a higher proportion of fat with skin compared with gilts (P=0.02). Interactions were observed between sex and half-carcass mass, so the sex of heavy half-carcasses becomes an important determinant for conditioning the muscle content. Equations were calculated and allow a fast and highly accurate determination of the lean meat content in bellies, suggesting they may be used directly in the production line.