Rainwater Harvesting Systems, Inc.

For every 1,000 square feet of catchment area you have, you can potentially collect 620 gallons of water for every inch of rain.

Remember:

• Only the fooprint of your catchment surface matters when calculating collection potential.  Roof slope is not considered in this equation.
• In reality, some water is always lost to splashing, evaporation, and roof washing. 
• The simplest way to calculate actual collection volume is to divide the square footage of your roof by 2.  So, a 2,000 sq. ft. catchment area will collect 1,000 gallons per inch of rain.