Rainwater Harvesting Estimate Calculator
Estimate annual harvestable rainwater from roof area, rainfall, and runoff coefficient.
Last reviewed by the Radiatus Cloud team
Output
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Estimate harvestable rainwater
Rainwater harvesting captures rain from a roof, and estimating the yield helps plan a system. This calculator estimates the annual harvestable rainwater from roof area, rainfall and a runoff factor.
Why roof area and rainfall drive the yield
The water you can harvest is essentially the rain that falls on the roof, the roof area times the annual rainfall, reduced by a runoff factor that accounts for what is lost to evaporation, splashing and inefficiency. A larger roof and higher rainfall obviously yield more, and the runoff factor keeps the estimate realistic rather than assuming every drop is captured. Estimating the annual yield shows whether harvesting is worthwhile for your situation and helps size storage, turning an eco-minded idea into a concrete figure to plan around.
A planning estimate
These are estimates from a standard method for general planning, not professional advice; confirm anything important for your specific situation. It runs entirely in your browser, so nothing you enter is uploaded and it works without an account, keeping your data on your own machine.
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Frequently Asked Questions
How is harvestable rainwater estimated?
As the roof area times the annual rainfall, reduced by a runoff factor for losses to evaporation, splashing and inefficiency.
What is the runoff factor?
A reduction accounting for the rain that is not actually captured, so the estimate is realistic rather than assuming every drop is collected.
What increases the yield?
A larger roof catchment area and higher rainfall. The calculator combines both with the runoff factor for the annual estimate.
What is the estimate useful for?
Deciding whether harvesting is worthwhile for your situation and sizing the storage needed to make use of the captured water.
Is my data uploaded?
No. The calculation runs entirely in your browser.
Privacy & Security
Calculated locally in your browser. Use local rainfall data for better accuracy.
How to Use
Enter roof area and annual rainfall to estimate harvestable water.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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