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Phil, Mechanical Engineer
Category: Home Improvement
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Experience:  Retired contractor, 51 years experience
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what is the formula for calculating the amount of water collectible

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what is the formula for calculating the amount of water collectible from annual rainfall off a flat surface (roof)

Welcome to Just Answer!

The formula is:

H is the published annual rainfall height in inches and A is the collection area in square feet, the amount in gallons is given by


(H/12) * A * 7.48052 = gallons collected per year,


(thanks to some coaching from one of our masters degree physics teachers)


We must subtract from that figure however depending on the rate of rain fall per event and wind conditions, which will evaporate small amounts of water from a roof that would not evaporate from a rain gage.


a 10% to 15% figure would be a reasonable estimate of loss due to evaporation.



. I hold questions open after positive ratings to allow for unlimited follow up. Please be patient though. The research & typing take time.

Please rate positively, Thanks!


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