Rainwater Catchment Calculator
See how much rain you can harvest off your roof. Enter the size of your collection roof and how much rain falls, and we'll estimate the gallons — per storm, per inch, and over a year.
Use the roof's footprint (length × width as seen from above) — the slope doesn't change how much rain lands on it.
Arizona's high desert often runs roughly 10–14 inches a year. Adjust to your area.
🌧️ Water you can collect
Roof catchment area—
From this storm—
Per 1 inch of rain—
Over a full year—
Rule of thumb: about 600 gallons per 1,000 sq ft per inch of rain. A metal roof and good gutters capture the most; first-flush diverters keep your stored water cleaner.
Estimates assume about 85% collection efficiency (real-world losses to splash, evaporation and overflow). Filter and treat collected water before drinking, and check local rules on rainwater harvesting. Planning estimate only.
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