Rainwater Harvesting Calculators are based on:
- Size of roof
- Amount in storage
- Daily / Monthly Demand (Inside & Outside)
Rainwater Supply & Demand
In order to calculate the amount of rainwater that can be stored you have to evaluate local data on rainfall such as the monthly and yearly average rainfall for a region.
A fantastic resource for calculating rainwater can be found at Texas A&M’s website: http://rainwaterharvesting.tamu.edu/