National Stormwater Management Calculator

This online tool compares predevelopment, conventional development, and development with Green Stormwater BMPs for a single site. Results include changes in total runoff volume, pervious area, and life cycle costs. Inputs include: site size, predevelopment land cover, post development land cover, and parameters for Green Stormwater BMPs. Default values can be used where information is not available. The calculator uses zipcode to determine average annual rainfall. This tool can also be used to find a combination of BMPs that will meet a particular runoff reduction goal.

https://greenvalues.cnt.org/index.php