Rainwater Harvesting Calculator
How much rain could your roof collect, and how far could that water go? Use local rainfall history to explore collection potential, tank sizing, and water supply through wet and dry months across the U.S. and beyond. Free to use.
New to rainwater harvesting? Learn the basics or explore the rainfall sources and calculation methods.
Help us improve rainfall coverage
Our new NASA POWER rainfall dataset is being tested. Tried it in your area? Share your feedback to help us improve the calculator.
We’re seeking funding to bring higher-resolution PRISM rainfall data to every U.S. state. Contact us about supporting this work.
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Find Your Location
Enter your address or click on the map to select your property and get location-specific rainfall data.
Rainwater Site Planner
Measure your roof catchment on aerial imagery, arrange multiple tanks, and compare round, rectangular, and slimline footprints before planning an installation.
Additional tools include custom rainfall analysis for your own data and a firm-yield calculator.
How much rainwater can your roof collect?
Collection depends on rainfall, the roof area connected to your gutters, and collection efficiency. A larger roof can collect more water, but your tank and water use determine how much of it you can keep and use. This calculator helps homeowners, gardeners, small farms, and system planners explore those tradeoffs.
Gallons collected = roof area (sq ft) × rainfall (in) × 0.623 × efficiency
For example, 1 inch of rain on a 1,000 sq ft roof at 80% efficiency yields about 498 gallons before tank overflow and water withdrawals. Efficiency is entered as a decimal: 80% = 0.8.
Choose a tank for dry periods, too
An annual rainfall total does not tell you whether water will be available when you need it. Compare tank sizes against historical wet and dry months and your expected demand. A larger cistern can reduce overflow and bridge some dry periods, but cannot make up for a persistent shortage of rainfall. Read the tank sizing assumptions before choosing a system.
Need a roof measurement or space for several tanks? Use the roof catchment and tank layout planner. For your own rainfall records, explore the custom rainfall and firm-yield calculators.
Rainwater calculator questions
Can I use the calculator outside Texas?
Yes. Select NASA POWER for locations across the U.S. and internationally where rainfall records are available. This option is being tested. PRISM is currently available in this tool for Texas only. Check the rainfall coverage and dataset differences for your project.
What roof area should I enter?
Use the horizontal footprint of the roof draining to the tank, rather than the sloped roof surface or the entire property. Exclude roof sections that drain elsewhere.
Where should I start with a rainwater system?
Learn how rainwater harvesting works, explore its benefits and tradeoffs, and decide which water uses fit your property. Our planning resources link to data providers and design guidance.
Who built this calculator?
The project was created by Wallace Darling, a hydrogeologist based in San Marcos, Texas. Read about the creator and project or send feedback about your results.