Area-capacity curves are of the most important physical characteristics of dam reservoirs. These curves are used for reservoir flood routing, reservoir operation, prediction of sediment distribution in reservoirs, etc. In this study, the linear characteristic of a reservoir’s depth-capacity data on log-log paper is used and the mathematical equation of the reservoir’s capacity-depth curve is obtained based on the shape factor, M. The reservoir’s area-depth equation and equation of shape factor are obtained by differentiating the reservoir’s capacity-depth equation. The obtained equations are evaluated with the area-capacity data of 8 reservoirs in the United States. The results of this study showed that the obtained equations agree well with the actual data.