In this paper, a simple design equation to predict the resisting capacity of circular CFT columns is introduced, and the accompanying design procedure entails two phases: construction of the linearized P-M interaction diagram for a circular CFT column section and its adjustment according to the slenderness ratio. To construct the linearized P-M interaction diagram of circular CFT columns without a rigorous nonlinear analysis, simple equations are proposed in this paper. In advance, on the basis of the numerical results obtained by the use of the analytical model introduced in the previous paper, the ultimate resisting capacity reduction factors are designed through a curvilinear regression, and the use of these reduction factors makes it possible to determine the ultimate resisting capacity of slender CFT columns without any rigorous nonlinear analysis. Finally, the efficiency of the introduced design equation is verified on the basis of the comparison of the ultimate resisting capacities calculated in this paper with those determined by rigorous nonlinear analyses and by the AISC design guideline and the Eurocode. (C) 2017 Elsevier Ltd. All rights reserved.