We developed the Stormwater Runoff Modeling System (SWARM) based on curve number and unit hydrograph methods of the U.S. Department of Agriculture, Natural Resources Conservation Service. SWARM models single events, targets watersheds fitting easily within hydrologic units with 12-digit codes, and has been calibrated for low-gradient topography of the Southeast coastal plain. We established protocols; made changes related to peak rate factors, travel time formulas, curve numbers, and the initial abstraction ratio; and then tested the output with multi-site validation using U.S. Geological Survey measurements of discharge and rainfall. Validation results from both undeveloped and developed watersheds support the robustness of our system in quantifying and simulating runoff: rainfall to runoff differences between measured and simulated volumes ranged from 3 to 11%; r2 for hydrograph curves ranged from 0.82 to 0.98. SWARM can be a useful tool for scientific research and for coastal resource management and decision making in the Southeast coastal plain specifically and also may be applied to other areas by recalibrating parameters and modifying calculation templates. Copyright © 2012 John Wiley & Sons, Ltd.