Rainwater harvesting calculators are important for several reasons. They help to better balance the supply and demand so that a system can be sized correctly and so the user has adequate storage to get through long periods without rainfall.
Below are two charts. The first shows the average rainfall in Southern Arizona and how many gallons of rainwater per month that can be collected from a 1000sq. ft. home. As you can see, a 1000 sq ft. home collects an astounding average of 7800 gallons of water a year with rainwater harvesting. The second chart shows the amount of water your home can collect based on the amount of rainfall and square footgage of your home. For example, a 2000 sq. ft. home collects about 1247 gallons of water with just 1″ of rainfall. How much water could you collect from your roof?