The Poisson’s rate value of the rock mass is one of the most important rock mechanical and rock engineering parameter. This value is used for calculating the deformation of the tunnels or displacement on rock foundations, among the others. Unfortunately, measuring this value is very difficult and time consuming in rock masses. The goal of this paper is to present a simple method for estimating the Poisson’s rate value for rock masses if it is known for the intact rock. Using the well-known equations for determining the earth pressure at rest, the relationship between the internal friction angle and the Poisson’s rate value can be determined. Due to the relationship between the internal friction angle and the rock mass classification (namely GSI) are also determined, from these connections the Poisson’s rate value and the GSI can be calculated. A linear equation was found: decreasing the quality of the rock mass, the Poisson’s rate is increasing. The presented calculation is also good, if the Poisson’s rate of the intact rock is not determinable. In this paper it was assumed that the Poisson’s rate value is a material constant. Note, that theoretically it is not true, however for rock mechanical calculations the changing this value is never investigated.