An enhancement ofMulti-box decks a method ofFlutter derivatives literature for the approximate calculation of multi-box girder flutter derivativesFlutter derivatives is proposed. The original method is based on the linear superpositionLinear superposition of flat plate flutter derivativesFlutter derivatives, where the plates width and distance from the elastic axis are embedded in the formulation. In the modified version proposed here, the flutter derivativesFlutter derivatives of each box are calculated according to Theodorsen’s flat plate theory, wherein the steady-state aerodynamic coefficientsAerodynamic coefficients of the flat plate are replaced by those of the actual boxes. The lift and moment coefficients of the different boxes are calculated by static CFDComputational Fluid Dynamics (CFD) analyses of the whole multi-box section. In this way, the wind flow variation induced by the upstream bodies is taken into account, albeit in a static manner. Three different cross-sections are investigated, for which the computed flutter derivativesFlutter derivatives are compared to the experimental ones, showing improvements with respect to those predicted by the original simplified method. In particular, a noticeable improvement is observed for the evaluation of the aerodynamic torsional stiffness. Eventually, the proposed formulationAnalytical formulation is tested for the flutter prediction of the 1915 Çanakkale Bridge, showing that the computed flutter derivativesFlutter derivatives provide a correct assessment of the coupled-mode flutter.