This paper presents the effect of soil-structure interaction on seismic inelastic displacement ratios of Single Degree of Freedom (SDOF) systems. Existing methods used in the past assumes the soil to be rigid. Through simplified equivalent fixed-base methods, the effect of soil-structure interaction (SSI) on the inelastic behaviour of structures is evaluated for different soil parameters using effective period and damping values. Using a degrading modified Clough model, the influence of different types of degradation is accounted for, for SDOF systems with periods ranging from 0.2 to 1.4 s. In total 300 different earthquake motions recorded on firm soil condition with magnitudes greater than 5 and peak ground acceleration (PGA) values greater than 0.08 g were selected. These records were scaled to the same hazard level and applied on five experimentally-tested reinforced concrete columns selected from the Pacific Earthquake Engineering Research Centre database. A total of 384,000 dynamic analyses were conducted. The results of the soil interacting systems are compared with the fixed-base case for different strength reduction factors (R) and foundation aspect ratios (h/r) for a range of NEHRP soil types C and D properties. These results show that the maximum inelastic displacements for soil type D are greater than those of soil type C and the fixed-base case. Particularly for periods less than 0.6 s with large aspect ratios, the effects of soil-structure interaction should be accounted for. Finally, the collected data was used to derive mathematical expressions for inelastic displacement ratios of SSI systems, suitable for use in performance-based seismic evaluation of structures.