This paper establishes a numerical detention pond volume model based on the hydrological continuity equation and the Runge-Kutta numerical method. Experiments for the conditions of both steady and unsteady flow have been used to verify the model. In unsteady flow cases, the outflow hydrograph by numerical simulation are fairly consistent with experimental value. Both experimental and numerical results indicate that wider rectangular sharp-crested weirs or larger rectangular slot tend to induce greater outflow discharges, which undesirably cut down the detention volume. Experiments show that the necessary detention volume of rectangular slot is smaller than that of the rectangular sharp-crested weir for a constant flood peak reduction. That is, the rectangular slot is the recommended outflow device when flood peak reduction is the design criteria. The study also shows that necessary detention volume of the short rainfall duration is less than that of the long rainfall duration under constant allowable maximum discharge. This paper establishes a numerical detention pond volume model based on the hydrological continuity equation and the Runge-Kutta numerical method. The unsteady flow have been used to verify the model. Both experimental and numerical results indicate that wider rectangular sharp-crested weirs or larger rectangular slot tend to induce greater outflow discharges, which undesirably cut down the detention volume. Experiments show that the necessary detention volume of rectangular slot is smaller than that of the rectangular sharp-crested weir for a constant flood peak reduction. That is, the rectangular slot is the recommended outflow device when flood peak reduction is the design criteria. The study also shows that necessary detention volume of the short rainfall duration is less than that of the long rainfall duration under con stant allowable maximum discharge.