The Federal Energy Management Program’s Rainwater Harvesting Calculator helps federal agencies estimate the amount of rainfall that can be harvested at locations across the U.S. A rainwater harvesting system captures, diverts, and stores rainwater from rooftops and other hard surfaces for later use. Potential uses of harvested rainwater include landscape irrigation, ornamental pond and fountain filing, cooling tower make-up, and toilet and urinal flushing.

This calculator estimates the amount of rainfall that can be collected at a given location by using historic precipitation data or user inputted precipitation data. The output of the tool provides the estimated gallons of rainwater that can be collected monthly for a given roof size, which can help to properly size the storage required by the system.

For more information on rainwater harvesting, see:

• Rainwater Harvesting Systems Technology Review: Key information to deploy rainwater harvesting systems
• Rainwater Harvesting Tool: Mapping tool showing state-by-state regulations and potential for harvesting rainwater across the U.S.
• Best Management Practices #14: Alternative water sources