Rainwater Collection Calculator

  • Look up your area's average annual rainfall totals here. For Austin, TX, the average annual rainfall is 32 inches.

Don’t know your roof area? We got you covered.

Enter your address below and you will be whisked off to the aerial view of your house.  Use the zoom tool in the lower right hand corner of the map to get closer so you can see draw on your roof.  Then click on the outer points of your roof and a polygon will instantly be created.

The area measurement will appear right above the map.  Once you have the square footage number, enter it into the rainwater collection calculator above.

If you want to look at collecting from different areas of your roof, you can always hit the “Clear Area” button to start over.

Rainwater Collection Calculation Formulas and Equations

Roof Area (ft2) X Precipitation Amount (in) X 0.623 = Amount Collected (gallons)

Easy Formula to Remember: 1″ of Rain on 1,000 sf Roof will Yield 623 Gallons

Roof Area (m2) X Precipitation Amount (mm) = Amount Collected (liters)

Other Water Conservation Calculators

Drip Calculator

An online drip calculator that calculates the amount of waste from your leaky faucets. A service by the American Water Works Association.

Rainwater Tank Performance Calculator

A tank performance calculator provided by the Domestic Roofwater Harvesting Programme at the School of Engineering, University of Warwick, England.

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