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FREE OFF-GRID TOOLS

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.

Why catchment works well here — and where it falls short

Apache County gets most of its yearly rain in short, intense monsoon bursts between July and September, which is actually good for catchment — a lot of water falls fast, and a well-set-up roof and gutter system can capture a real amount off a single storm. The catch is the rest of the year: winter and spring can go weeks without meaningful rain, so catchment alone rarely covers a full year of household use out here. Most people treat it as a supplement to hauled or well water, not a replacement for it.

Metal roofs vs. shingles

Metal roofing sheds water fast and doesn't shed grit the way asphalt shingles do, which is why most serious off-grid catchment setups out here use a metal roof if they have the choice. If you're working with shingles, a first-flush diverter (which dumps the first few gallons of a storm, along with most of the dust and debris) makes a bigger difference for water quality than almost any filter you could add downstream.

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