The total amount of water harvested depends upon the roof catchment area and the average annual rainfall in that region.
For example: For 100 SQM catchment area with average annual rainfall of 2000 mm has the following calculation:
Total Amount of Water Harvested = Area of Catchment x Average Annual Rainfall x Coefficient of Transportation x Coefficient of Filtration
Total Amount of Rainwater Harvested = 100 x 2000 x 0.90 x 0.85
= 1, 53,000 Ltrs per Year.