Infill-walls, commonly used as space-separating components, are present in most structures. Although considered non-structural elements, they significantly influence the seismic response of buildings by altering their lateral stiffness and strength. Several approaches have been proposed for modeling infill-walls, among which the equivalent strut method is the most widely adopted in structural design codes. In this method, the infill-wall is represented by a diagonal compression member. Numerous researchers have introduced different expressions for estimating the effective width of the equivalent strut; however, their accuracy and applicability remain limited. This paper presents a new method for determining the stiffness of the equivalent strut. Based on the proposed algorithm, the strut stiffness is obtained for both linear and nonlinear ranges such that the pushover curve of the structure containing the strut, exhibits the highest possible agreement with that of the structure with the infill-wall. A computer program was developed in the MATLAB–ABAQUS environment to implement this algorithm. After validating the performance of the software, it was used to calculate the equivalent strut properties for two experimental specimens. The results indicate that the accuracy of the strut behavior predicted by the proposed method is significantly higher than that of previously published relationships. The maximum deviation in estimating the pushover curve, even in the nonlinear phase, was limited to 6%, demonstrating that the proposed method can be reliably used to determine the equivalent strut properties.