Rainwater Harvesting Calculator

Rainwater Harvesting Potential

Rainwater harvesting captures precipitation from rooftops for later use. It reduces stormwater runoff, lowers water bills, and provides a backup water source during droughts. Even modest roof areas in moderate rainfall regions can collect thousands of gallons per year.

The formula is straightforward: multiply roof area in square feet by annual rainfall in inches, then by 0.623 to convert to gallons, and finally by your collection efficiency percentage. Efficiency accounts for losses from evaporation, splash, and overflow, typically 75-85%.

Collected rainwater is excellent for garden irrigation, lawn watering, car washing, and toilet flushing. Tank sizing should account for your dry season. A tank holding 1-2 months of collection volume ensures water availability during rainless periods.