Rainwater Firm-Yield Calculator

How much water can your roof reliably supply?

RAINFAL runs a daily water-balance across decades of local rainfall to show whether a system would have met demand every day — even through the worst drought on record.

84+
Years of rain data
20
Texas stations
Daily
Water balance
Start designing

System design

Roof or other surface that catches rain, in square feet.
How much water your tank can hold, in gallons.
Water used per day. A water-efficient home uses about 25 gallons per person.
Share of rain that reaches the tank.
Adjusts the local rainfall up or down. 1.00 = records as-is; 1.10 = 10% wetter, 0.90 = 10% drier.
How full the tank is when the simulation begins, as a percent.
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Reliability
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Avg days dry / year
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Lowest storage
Tank storage over the record