The present paper assesses the ultimate bending moment of steel box girders subjected to intensive non-uniform corrosion degradation. A comparison between the experimental results of three corroded box girders and different existing formulations has been performed. The moment–curvature relationship for the three corroded box girders has been calculated using the Common Structural Rules iterative process and the finite element method, leading to a development of corrosion scale factors to account for both geometrical and mechanical properties changes. Based on the experimental results, a relationship has been developed to calculate the equivalent Young's modulus of corroded structures as a function of the total reduction in cross-sectional area. A systematic procedure for the application of the newly developed fast approach and the new equivalent Young's modulus has been presented that can be used for any ultimate bending moment–curvature calculation tools.