Math CentralQuandaries & Queries


Subject: Calculating Rainwater Volume
Name: Mark
Who are you: Parent

Trying to calculate 1" rainfall on a 1000 sq' roof=how many gallons of water would collect in our cistern. Thank you for your assistance. Mark

Hi Mark.

If you have a flat roof, you can just convert units: 1000 square feet is 144 000 square inches. When that is 1" deep, you have 144 000 cubic inches of water. If you are using the american liquid gallon, there are about 231 cubic inches per gallon, so for your roof, you would get 623 gallons of water.

If your roof is not flat, you need to calculate the square footage of the ground it covers instead of the surface of the roof itself and use that rather than the 1000 sq feet.

Hope this helps.
Stephen La Rocque.

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