The duration of strong ground motion is widely recognized as an important characteristic affecting the response of man-made structures. The basis of this study is to define strong-motion duration in a manner consistent with the use of the root-mean square acceleration as a ground motion parameter. The root-mean square and duration give a good measure of the strong-motion intensity. The dependence of the root-mean square on duration is studied through the use of the cumulative RMS (root-mean square) function. The rate of change of the RMS is obtained by taking the derivative of the cumulative RMS function. The derivative identifies the time after which the RMS is always decreasing. This time value is used as the upper cutoff time (T2) in defining the significant duration (T2 − T1). The lower cutoff time (T1) is found by performing the above operation on the record with a reversed time scale. Results for the horizontal components of 30 strong-motion earthquake records are presented with the RMS for the defined strong motion. Comparisons of strong-motion duration as defined by the methods of Trifunac-Brady, Bolt, and Vanmarcke and Lai, and the proposed method are presented. The use of the root-mean square acceleration as a ground-motion parameter is also discussed.