The National Stormwater Calculator is a simple to use tool for computing small site hydrology for any location within the US. It estimates the amount of storm-water runoff generated from a site under different development and control scenarios over a long term period of historical rainfall. The analysis takes into account local soil conditions, slope, land cover and meteorology. Different types of low impact development (LID) practices (also known as green infrastructure) can be employed to help capture and retain rainfall on-site. The calculator’s primary focus is informing site developers and property owners on how well they can meet a desired stormwater retention target. It can be used to answer questions such as the following: · What is the largest daily rainfall amount that can be captured by a site in either its pre-development, current, or post-development condition? · To what degree will storms of different magnitudes be captured on site? · What mix of LID controls can be deployed to meet a given stormwater retention target? The calculator seamlessly accesses several national databases to provide local soil and meteorological data for a site. The user supplies land cover information that reflects the state of development they wish to analyze and selects a mix of LID controls to be applied. After this information is provided, the site’s hydrologic response to a long-term record of historical hourly precipitation is computed. This allows a full range of meteorological conditions to be analyzed, rather than just a single design storm event. The resulting time series of rainfall and runoff are aggregated into daily amounts that are then used to report various runoff and retention statistics.