This research is aimed at investigating the elastic buckling strength of I-beam cantilevers. In the section classification and in the design both, the understanding of the load that the beam can withstand without loss of stability is crucial, especially in determining the correct method of analysis. The section of the beam is divided into segments which are investigated separately as plates. A novel method of buckling strength approximation for plates with flexible boundary conditions is proposed, with respect to their lengths. The paper concludes with a proposal of an equation to estimate the buckling strength of the whole beam.