As a general rule, for every 1,000 square feet of catchment area*, you will collect 600 gallons of water per inch of rainfall.
Serving the greater Dallas-Fort Worth Metroplex
* The catchment area of your home is larger than the square footage of your floor plan and will include garage and covered deck areas, plus the amount of overhang your eves have. Also, the vertical slope of your roof does not increase your catchment area so only the "footprint" of the roof should be considered.
To see an example of a rainwater harvesting calculator, click here. The file will open as a MS Excel spreadsheet and is based upon average rainfall for Dallas, TX.
(Curtesy of the Texas Water Development Board)