A simple method is developed for calculation of inelastic response spectra for arbitrarily predetermined values of ductility and damping. Period-dependent Ductility & Damping Reduction Factor (DDRF) is defined as a scaling factor that convert 5%-damping elastic response spectra into an inelastic response spectra for desired damping and ductility. It is formulated as a product of two functions interrelating the ductility, damping and natural period, and a period-band-dependent matrix of regression coefficients which are statistically quantified on the basis of Japanese strong motion data set. Influence of site effects on DDRF is studied by classifying the strong motion records according to the soil condition classification employed in the Japanese Earthquake Design Specifications for Highway Bridges. On this basis a closed-form statistical model is proposed for prediction of a site-dependent inelastic response spectra for a given soil condition, ductility and damping.