Rainwater Harvest Calculator
Find your rainwater harvest potential.
Result
How to use
- Enter your values in the fields above.
- Press Calculate to see your result instantly.
- Use the Share button to copy a link to your result.
About this calculator
This calculator estimates how much rainwater a roof can harvest, using the standard formula volume = roof area × rainfall × runoff coefficient, where the runoff coefficient (typically 0.8–0.9 for a hard roof) accounts for water lost to evaporation, absorption, and overflow before it reaches a collection tank.
Homeowners planning a rain barrel or cistern system use it to size the tank correctly, and gardeners and sustainability-minded households use it to estimate how much irrigation water they could realistically collect off a given roof area over a season.
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