Urban stormwater managers have traditionally used pipes, ditches, ponds and other gray infrastructure elements to quickly divert runoff away from its main sources—buildings and roadways. In contrast, proponents of green infrastructure attempt to manage stormwater near its origin, utilizing natural drainage pathways and best management practices (BMPs) to reduce runoff and increase infiltration. In doing so, stormwater is retained where it is needed to support urban vegetation. This vegetation, in turn, helps reduce future runoff, while producing a whole range of environmental, economic, and social/human health-related benefits. Despite the many advantages of green infrastructure, retrofitting the infrastructure of a city is a costly process that requires careful planning. The transition from gray to green infrastructure requires communication between managers from different disciplines and a willingness to stray from management strategies that have defined stormwater management for centuries. The Gray to Green (G2G) green infrastructure planning tool is designed to facilitate these conversations—showing both technical and non-technical users how green infrastructure BMPs can work within the urban forest to manage stormwater on existing or proposed development sites. This paper details the data sources and research at the core of G2G—documenting all methods, equations, and assumptions used in its creation to provide users with a fully-transparent and peer-reviewed planning tool. The paper concludes with descriptions and user insights from two case studies from Tampa, Florida (United States) and Milwaukee, Wisconsin, (United States).