Rain Barrel Calculator

A 1,000 sq ft roof collects about 623 gallons per inch of rainfall. With 40 inches of annual rainfall, that is about 25,000 gallons potential per year, though actual collection depends on storage capacity and usage.

A standard 55-gallon rain barrel fills up from just 1 inch of rain on about 90 sq ft of roof. For a typical downspout draining 300-500 sq ft, you will need 2-4 barrels to capture a 1-inch rain event. Consider a larger cistern (200-500 gallons) for serious harvesting.

Rainwater harvesting is legal in most US states, though some states have restrictions. Colorado, for example, limits collection to two 55-gallon barrels. Check your state and local regulations before installing a system.