Today, mechanical engineering is necessary to use more precise equipment, for that reason calculation of the deformation of machine elements request higher accuracy. There is an elastic deformation of machine elements described in this paper taking into consideration the height of surface roughness. This articles studies and compares three different formulas for the calculation of the density of probability distribution of surface peak height and determination of the mathematical expectation value (ME) of surface peak height for those roughnesses being above a determined deformation level γ. In the contact theory surface is being modelled as a normal random field. The law on the density of probability distribution of peak height for such a normal random field was established by P.R. Nayak, yet this expression is practically inapplicable for the solution of engineering tasks, therefore the present work establishes that the given formula can be substituted by a simpler distribution law. The article treats the P.R.Nayak formula and two most well-known probability distribution laws: the normal distribution (Gauss) law and Rayleigh law. Comparing these three formulas a simpler, yet sufficiently precise solution was established which can substitute the complicated formula. The work clearly shows and demonstrates graphically the numerical values of the density of probability distribution of obtained surface peak height and the ME values of surface peak height at different γ values, for all three formulas; the table also summarises maximum deviations of the above mentioned probability distribution laws from the precise formula.