A simple hand calculation based method is presented to develop axial flexure design P-M interaction curves of rectangular reinforced concrete sections. The proposed method uses basic principles of mechanics satisfying compatibility of strains, equilibrium of forces and constitutive relations of constituent materials. Simple step-wise calculation is enough to develop the interaction curve; it does not require any iterations. Accuracy of the method is demonstrated by comparing the results of two RC sections with the interaction curves of the sections obtained using SP:16. Step-wise calculations are presented of the two RC sections to illustrate the use of this method for generating design P-M interaction curves.