Knowing the effective impervious area (the impervious area directly connected to the storm sewer system) is critical for more accurate stormwater modeling. This report provides a practical method to determine EIA using readily available data rather other more time-consuming and expensive methods. The new method uses existing geographic information system data and Natural Resources Conservation Service data, and primarily relies on curve number, which is a parameter used to determine runoff that integrates soil type, infiltrations rates, land cover and precipitation data.