Rainfall Collection Calculator

How can I figure out how much rainfall is needed to fill my barrels?

The easy way: A calculator

• 1 gallon = 231 cubic inches ≈ .1337 cubic feet
• 1 square foot = 144 square inches
• Surface area of the roof draining into barrels (Hint: you do not need to take into account the pitch, or slant, of your roof. You do have to take into account what fraction of your roof is emptying into rain barrels.For example, if you have a simple roof, say a rambler style house, and all the downspots in the back empty into rainbarrels, then the surface area emptying into rain barrels would be approximately 1/2 the width of your house times the length of your house. If you have two downspouts in the back and only one empties into a rain barrel, than it might be more like 1/4 the width times the length.)

         (total capacity of barrels in gallons x 231)
---------------------------------------------------- = # of inches of rainfall to fill
 (surface area in inches of roof draining into barrels)

Example:

• My house is about 60 feet long by 25 feet wide and half my roof empties into rain barrels, so the surface area in inches is 60 x 25 x 1/2 x 144 = 108,000 square inches
• I have 4 rain barrels at the backyard downspouts and they are 55 gallons each, so 4 x 55 = 220 gallons

So, (220 x 231) / (108,000) = 0.47 inches of rainfall needed

...that's less than a half inch of rain to fill my rain barrels with 220 gallons of water!

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All text and images © 2005-2008 Rebecca Chesin

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