Simple Rainwater Harvesting Calculator
How much rainwater can you capture is one of the easiest questions to answer when it comes to rainwater harvesting.  The math is very straight forward:

1) Enter how much does it rain in your area annually and
2) Enter how big is your catchment area (e.g. roof)
3) Multiple these two numbers together
4) Multiple the result by .623.

This formula calculates the maximum you can harvest. For those interested, .623 is the number of gallons that 1" of rain will provide on one square foot of surface area. 
  Input Annual Rainfall in Inches   <== Enter Number  
  Input Area Square Footage   <== Enter Number  
  0.623 Gallons Conversion  
  Total Possible Gallons Harvestable  
The above gives you an idea on how much you can capture.  So how do you find out your annual rainfall is?  How do you find out your square footage? The links below will help you locate this information.      
1. Weatherbase Rainfall Finder:   <==  Click on tool, enter zip code in search box, locate your locale on the list and find your annual rainfall on the table
2. Google Area Finder:   <== Click on tool, find your house, move to Satellite view and outline the area of your house.
Remember this is the maximium you can capture.  It is the best possible amount.  In reality you will capture less.  Your roof type and your conveyance  system (i.e. the water conveyance from roof to tank) will all effect how much you can capture.        
Go to FREE Online Site Analyzer that provides even more information