Free Rainwater Harvesting & Cistern Sizing Calculator
Calculate total roof catchment rainwater yield (gallons & liters), first-flush diversion, and optimal cistern tank storage capacity.
🌧️ Roof Catchment & Rainfall
📊 Harvest Yield & Cistern Size
Recommended Tools & Equipment
Tested hardware and components for high reliability
Frequently Asked Questions
How much water can I collect from a 2,000 sq ft roof?
In a region receiving 36 inches of annual rainfall, a 2,000 sq ft metal roof (0.95 efficiency) will capture approximately 42,600 gallons (161,000 liters) of clean rainwater each year.
What is the best roof material for potable rainwater harvesting?
Standing seam metal (coated with non-toxic epoxy or Galvalume) is the premier roof surface. It is smooth, non-porous, does not leach petrochemicals, and quickly sheds debris during the first flush.
How large should a first-flush diverter be?
The standard rule is 1 to 2 gallons of diversion per 100 square feet of roof area. For a 2,000 sq ft roof, size your first-flush pipe assembly to divert approximately 20 to 30 gallons.
Can rainwater be used for drinking (potable water)?
Yes, if properly filtered and disinfected. A complete potable treatment train requires a coarse leaf screen, first-flush diverter, 5-micron sediment filter, activated carbon block, and a certified Ultraviolet (UV) disinfection sterilizer.