Grassed swales or drainage ditches are an attractive BMP because they can reduce runoff volume by infiltrating water into the soil, filter sediments and associated pollutants out of the water, and settle solids onto the soil surface. Infiltration rate is a controlling factor for the water quality performance of swales. In this study infiltration measurements were taken at 108 locations in a swale that is located in Madison, Wisconsin. A model has been developed to estimate the infiltration of direct rainfall and roadway stormwater runoff into the soil of that swale for observed rainfall events and thereby facilitate the calculation of volume and rate of outflow from the swale. This model is based on the Green-Ampt equation for the infiltration calculation. The model takes into consideration the antecedent moisture condition of the soil and the Green-Ampt parameters for the soil. With this model the outflow rate of the swale was estimated for the monitored rainfall intensity of a selected storm using the saturated hydraulic conductivity measurements that were taken in the field and compared with the actual outflow rate from that swale. This paper will show how the measured infiltration parameters can be used to determine the stormwater mitigation performance of a given swale.