This article presents a simplified approach to quantifying the comparison of acceleration responses of high-speed craft in rough seas. Statistical acceleration values, used to characterize craft seakeeping responses, including average of the highest one-third, one-tenth, and 1/100th peak accelerations and the root mean square acceleration, are used to define the relative Ride Severity Index (RSI). The article first summarizes an unambiguous computational procedure for multiple investigators to calculate similar acceleration values. It then explains the theory and rational for relating statistical acceleration ratios to an indication of potential damage, whether resulting from cumulative wave impacts or single severe slam events, that can be used in comparative assessments of structural integrity, equipment susceptibility to malfunction, or personnel comfort and safety. Example ride severity comparison plots and computed values of RSI are presented to illustrate the simplicity of the approach and to demonstrate the ability to quantify what heretofore has relied primarily on the subjective experience of operators.