Although the hydrological continuity equation can be applied to the flood routing of a detention pond, the engineering design mostly adopts simplified models to calculate the maximum detention volume limited to the small design scale. Based on the numerical model developed by the hydrological continuity equation, this study establishes several graphic methods and a regression method to estimate the maximum storage volume of a detention pond, which are fairly consistent with the numerical solution. Therefore, these simple and convenient methods can be used in engineering design. In addition, this study shows the maximum storage volume using an orifice as the outflow device is smaller than using a weir as the outflow device. A long recession time will require small detention volume.