Long-term retention performance is a common performance indicator for low-impact development practices, such as rain barrels, rain gardens, and green roofs. This paper introduces a numerical approach for the estimation of annual retention ratios of stormwater by bioretention. The annual retention ratio is taken as the ratio of the annual accumulated volume of stormwater retained by bioretention over the total volume of runoff draining into the system. The hydrologic model Storm Water Management Model (SWMM) is used to simulate the relevant flows of a bioretention system with parametric variations of the watershed area ratio and hydraulic conductivity of the soil media. Under these two dominant performance-governing parameters, retention ratios are calculated using the 10-year (2004–2013) rainfall record in Hong Kong at 1-min intervals. This indicator can be readily applied to estimate the long-term retention performance of a bioretention using particular values of watershed area ratio and hydraulic conductivity of soil media under the climate of Hong Kong. The study also analyzes the influence of variation of annual precipitation on the estimated retention performance. Flow data monitored on a pilot-scale physical model of bioretention during a number of rainfall events are used to validate the numerical simulation.