My Rain Barrel Project Make Your Own Rain Barrel FAQ Calculate Your Rain Harvest Is Roof-Collected Rain Safe? About

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Rainfall Collection Calculator

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

The easy way: A calculator

Capacity of a rain barrel gallons
Number of rain barrels
Dimensions of house x feet   (your house is ??? square feet)
Portion of roof emptying into rain barrels    (enter as decimal, not fraction--half the roof would be .5)
Rainfall needed to fill barrels ??? inches
The harder way: Nitty-gritty details
  • 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.)
The equation:

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


  • 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!


Now that you know how much rain you can collect, you might
want to know where it's use is recommended or discouraged.

My Rain Barrel Project Make Your Own:
How to Construct a Rain Barrel
FAQ Calculate
Your "Harvest"
But Is It Safe? About


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Happy Watering!


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