The U.S. Environmental Protection Agency (EPA) is considering a national stormwater regulation that would require all newly developed or re-developed construction projects to maintain some measure of predevelopment hydrology for the developed site. As part of this effort a National Stormwater Calculator tool is being written to help developers and site planners determine a runoff target that complies with the definition of predevelopment hydrology and show what mix of management practices and low impact development (LID) alternatives could be used to meet the target. The Calculator provides a user-friendly front-end for running EPA’s Storm Water Management Model (SWMM) under a long term continuous rainfall record for both predevelopment and post-development conditions. These two conditions represent different mixes of land cover (forested, grass, impervious, etc.) and LID controls (disconnection, conversion, and treatment) applied to the site. A statistical post-processor is used to compute various metrics of the site’s hydrological response under both types of conditions. A unique feature of the Calculator is that it seamlessly accesses several national data bases to retrieve precipitation, evaporation, and soil type data based just on the site’s location (either lat-long or zip code). The Calculator provides a more technically defensible approach to defining predevelopment hydrology than one based on simply controlling a single pre-determined design storm event.