Analysis of the texture profile of surfaces machined by different methods shows that the geometric parameters of the surface microprojections do not fully characterize their shape, repetition frequency, height, and other properties. Instead, a potential comprehensive characteristic is the filling probability of the rough surface layer over the thickness and the volume as a whole. This parameter is shown to correspond to the actual quantitative estimate of the microprojections bounded by analytical curves such as convex semicircles. The analytical expressions obtained may be compared with the probability characteristics of surface roughness bounded by other curves specified analytically or from profilograms.