Guided waves, especially Lamb waves or shear-horizontal waves, are widely used types of waves for ultrasonic inspection of large structures. Well known property of guided waves is their dispersive character, which means that the propagation velocity of the particular wave mode is not only a function of physical properties of the material, in which the wave propagates or the wave´s frequency, but also depends on the geometry of the structure in itself. Dispersion curves provide us information related to the dependency between the wavenumber and the frequency of the particular mode. Dispersion curves are the product of numerical solution of Rayleigh-Lamb frequency equation, which can be realized in order to obtain real, imaginary or complex wavenumbers. A solution of Rayleigh-Lamb frequency relation forms a set of a finite number of real and pure imaginary wavenumbers and an infinite number of complex wavenumbers. Proposed paper presents a complete procedure of how to obtain all three kinds of wavenumbers for a given geometry. The main emphasis is placed on the effectiveness of the algorithms, which are used for finding the roots of dispersion equation for all three kinds of wavenumbers.