The objective of this study is to develop inelastic displacement ratio for calculating peak inelastic displacement of SDF systems. The inelastic displacement ratio is defined as the ratio of peak displacement of inelastic SDF system to the peak displacement of corresponding elastic SDF system. Statistical studies are carried out to estimate median inelastic displacement ratios and those dispersion of bilinear SDF systems with given yield-strength reduction factors, natural periods, post-yield stiffness ratios and damping ratios ranging from 1 to 20% when subjected to 60 earthquake ground motions recorded on stiff soil sites. This study shows that equal displacement rule overestimates the peak inelastic displacement of the system which has a fundamental period ranging from the intermediate to long period. It is also observed that no significant difference was observed in the value of median inelastic displacement ratio across site classes.