One rule of thumb I’ve come across is: for every 1,000 square feet of roof, one inch of rainfall can yield about 600 gallons of water. Alternatively, this formula (which can be found on many websites) can be used:
Area of roof in sq. ft. X inches of rainfall per year X 0.62 = gallons runoff per year
(Here’s a unit analysis for that formula: ft X ft X in/year X ft/12 inch X 7.48 gal/cu.ft. So, 7.48/12 is where the 0.62 comes from.)
So for my ~1,200 square foot house, using an average of 29.7 inches of rain per year, I come up with roughly 22,000 gallons of harvestable water! That’s a big tank!
1200 sq. ft. X 29.7 in./yr. X 0.62 = 22,096.8 gallons.
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