The microgeometry of cut surfaces is one important characteristic of surface quality, since it has significant influence on the tribological properties of working surfaces. In addition to the ability to plan the economical effectiveness of cutting and the accuracy of workpieces the ability to plan the quality of machined surfaces has also emphasized importance. This is why the predictability of surface roughness in planning of technological processes is more and more important. More and more opportunities presenting themselves and one of these is the estimation of the expected value of roughness of the machined surface on the basis of the theoretical value of roughness. The paper focuses on the introduction of a method to describe a mathematical relation between the theoretical and the measured roughness indexes in cutting with tools having defined edge geometry, thus the planning may be more accurate.