This paper proposes a simplified method to design slender rectangular reinforced concrete columns with doubly symmetric reinforcement subjected to combined axial loads and biaxial bending, which is valid for short-time and sustained loads and for both normal and high strength concretes. The method is based on the so-called moment magnifier method (ACI-318-02) and EuroCode-2 (draft for stage 49) and it is only suitable for columns with equal effective buckling lengths in the two principal bending planes. A new equation of the column stiffness EI is proposed with which to obtain the moment magnification factor for single and double curvature. The method was compared with 371 experimental tests from the literature and a good degree of accuracy was obtained. Precision for sustained loads was improved in comparison with the method proposed by the American ACI-318 (02) and the European EC-2 (September 2003). The method allows slender reinforced concrete columns to be both checked and designed with enough accuracy for practice engineering.