We propose a method to compute the axial distribution of non-diffracting beams. The approach stands on simple ray tracing and energy conservation principles. The proposed method is applied for four different Bessel beam generators, such as the refractive axicon, to compute the output axial intensity profile for a given input beam. We show great agreement between the results and the CODE V diffractional simulations. The method can also be used to calculate analytically the incident illumination pattern needed for a target output profile or to design an optical element surface to reach the same goal.