Rainwater Harvesting Calculator
Estimate the rainwater you can capture from a roof. Enter the roof catchment area in square feet and the rainfall in inches to see gallons and liters.
How to estimate rainwater harvest
- Enter your roof catchment area in square feet (the footprint).
- Enter the rainfall for the storm or period, in inches.
- Set a collection efficiency, often about 0.75 to 0.9 in practice.
Examples
1000 sq ft roof, 1 inch of rain
Roof 1000, rain 1 in, efficiency 1
623 gallons, about 2358.31 liters captured
Frequently asked questions
How much water comes off a roof per inch of rain?
About 0.623 gallons per square foot per inch. One inch over a square foot is one twelfth of a cubic foot, and a cubic foot holds roughly 7.48 gallons.
Should I use the roof area or the footprint?
Use the footprint, the area the roof covers as seen from above. A steeper pitch does not catch more rain, since rain falls onto the projected, flat area.
What collection efficiency should I assume?
Around 0.75 to 0.9 is realistic. Water is lost to splash, evaporation and first-flush diversion that discards the dirty initial runoff. Smooth metal roofs do best.
How big a tank do I need?
Size it to your typical storm and your use between rains. Multiply your roof area, a common rainfall and the efficiency to see how fast a tank fills.
Is harvested rainwater safe to drink?
Not without treatment. Roof runoff can carry debris, bird droppings and contaminants. It is fine for irrigation, but drinking needs filtration and disinfection.
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