You may not realize it, but 623 gallons of water fall on an average 1,000 sq. ft. residental roof, during a period of one-inch of rain * ! With a system that is 80% efficient at catching rainwater, the amount of rainwater that can be collected is significant. Assuming 30" of annual rainfall (or 2.5" per month**), 2,000 gallons of rainwater per month can be harvested using the Mystic™ Rainwater Collection System! Our “Mystic Rainfall Calculator” allows you to calculate the average rainfall of a specified location in a fun and interactive format! The calculator provides both monthly and annual averages for a variety of major cities across America and Canada. Simply select the city closest to the preferred geographic location, along with the desired month, to see how many inches of rain falls in that area (on average).   * Based on United States Geologocal Survey data at http://ga.water.usgs.gov/edu/earthrain.html ** National Oceanic and Atmospheric Administration, State of the Climate, National Overview, Annual 2008.