The strength reduction factor presented in design codes has been examined disregarding the influence of soil-structure interaction (SSI). However, it has been proved that SSI can significantly affect structural responses. Most recent studies evaluating the strength reduction factors have primarily focused on single-degree-of-freedom soil-structure systems, potentially overlooking the behavior of multi-degree-of-freedom systems. Furthermore, there is a lack of research, concerning the suggestion of a proper relationship to estimate the strength reduction factor of multi-story buildings considering SSI. Therefore, this paper focused on investigating the SSI effects on ductility strength reduction factor (Rμ ) and presenting a simplified expression to practically estimate Rμ, including inertial and kinematic interactions. To obtain reliable results, several nonlinear multi-story structures with different ductility ratios subjected to earthquake ground motions on soft soils were analyzed. Unlike other studies that utilized simple spring methods to model the soil, this research represented the soil as a continuous medium. Consequently, this study addressed the limitations of modeling found in comparable research and, building on this, introduced a novel relationship for application in design codes. By comparing with the literature review, it was noted that the suggested formula could provide a fairly accurate estimation of the strength reduction factor.