A correct estimation of the methods for calculating the sedimentation area can be performed, in principle, only on the basis of experimental tests on industrial-scale settling tanks. Although more than 80 years have passed since the fundamental work of Coe and Clevenger was published, there are surprisingly few works concerning this problem [2]. In our experimental studies we have utilized industrial-scale settling tanks, employed for the thickening of coal and magnetite suspensions. Such apparatuses operate in coal mines. The investigations comprised determination of now rates of suspensions and sampling of suspension in order to determine the concentration of solids and to carry out sedimentation tests. The former have been determined by the gravimetric methods after evaporation of liquid. The initial concentrations amounted to 30-378 kg of solids/m(3) of suspension and the outlet concentrations were in the range of 207-830 kg of solids/m(5) of suspension. The sedimentation tests have been performed in glass tubes of 0.04 m in diameter and of the lengths of 0.8 or 0.95 m. On the basis of the dependence of measured heights of interface vs. time the sedimentation curves of the tested suspensions have been plotted. Each curve has been drawn on the basis of ca. 80 experimental points. Next, the points of compression of tested suspensions have been determined. To this end the diagrams of Roberts and Fitch were used. The experimental investigations as above and the graphs served to determine the sedimentation area according to the methods described in [1], chapter 3. The calculated areas have been compared with the actual areas of industrial thickeners, and the results are presented in Table 1. The comparison of the methods of calculating the sedimentation area has been carried out for the suspension of technical calcium carbonate. Then it was possible to determine the dependence of specific sedimentation areas on concentrations phi(2) of thickened suspension, and in the case of Font method, on sediment concentrations phi(2)(0) (Fig. 3). The methods of calculation of sedimentation area have been divided into two groups, depending on the way of utilization of the sedimentation; rests. The first group comprises the methods making use of the diagram of nux density of solids, and the other - the methods utilizing directly the sedimentation curve. As a comparative criterion for the first group, the minimal total volumetric nux density of solids, v(x, min) , has been assumed, and for the second group - the characteristic time for sedimentation under given conditions. it das been shown that the values of the minimal total nux density of solids, calculated by the methods of Coe and Clevenger, Yioshioka, as well as Merta and Ziolo are identical and a corresponding diagram (Fig. 5) has been drawn. It has been found that the inequality t(Ol)tOl). Summing up, it is advisable to employ the methods of the first group in calculating the sedimentation area for inorganic suspensions. In particular, this can be done in the simplest and quickest way on the basis of one sedimentation test, making use of the method of Merta and Ziolo. On the other hand, for organic suspensions (no active; sludge) the method of Oltmann or Talmage and Fitch may be used. In the group of activated sludges, a considerable scatter of experimental results occurs and the calculated sedimentation area, as it results from the average value of F-act/F-calc, may be four times smaller or by 40% greater than the actual one.