The Original Ramamurthy et al. (1985) criterion propose the estimation of rock strength under compression forces. This criterion includes two constants B and α, which account for properties of different rocks. The validity of the strength criterion depends on the number of the data used to extract these constants. In this paper a wide range of triaxial lab data corresponding to various rock types reported in the literature was used to determine constants B and α in Ramamurthy’s failure criterion. In addition these constants were determined for coal using cores taken at different angles with respect to its foliation planes: this allowed taking the isotropic transverse behaviour of coals into account. To fit the best curve to the data both linear conversion model (LCM) and nonlinear regression model (NRM) were applied. The results of the analyses and the proposed constants for different rock types and coals are presented.