How much rainwater can you collect from your building roof?

Rainwater potential from your house roof

Roof area in sq. m.   -  A

Annual Rainfall of the place in mm - R

Potential of Rainwater that can be Harvested

  = Area X Rainfall = A X R = …… liter

or

 Roof area in sq. ft.  -  a

Annual Rainfall of the place in inches - r

Potential of Rainwater that can be Harvested

  = Area X Rainfall = a X r X 2.36 = ….. liter

Example - For a house in Bangalore:

Roof area in sq. m.   -  100 
Annual Rainfall of the place in mm (Annual rain fall in mm for Bangalore) - 1000
Potential of Rainwater that can be Harvested
  = Area X Rainfall = 100 X 1000 = 1,00,000 liter
or
 Roof area in sq. ft.   -  500 
Annual Rainfall of the place in inches (Annual rain fall in inches for Bangalore) - 40

Potential of Rainwater that can be Harvested
  = Area X Rainfall = 500 X 40 X 2.36 = 47,200 liter