How many gallons of water can be captured from a tin roof measuring 4 ft x 8 ft, if you get a 1 inch rain? Converted into gallons?

I want to build a water guzzler, but need to know the size of area of my tin roof guzzler in order to capture 60 gallons.

  • 1
    What is the pitch/slope of the roof? The answers below are for a dead flat 4x8, which would not drain well - though for low slopes, the difference is not particularly significant. Or; if the structure below is 4x8, the pitch does not matter since the projected area is 4x8, even though the roof is then larger in the sloped direction.
    – Ecnerwal
    Commented Sep 12, 2023 at 15:12
  • 1
    I think there are a few imperial units you haven't mentioned yet! :-) Commented Sep 13, 2023 at 0:12
  • 1
    Switch to metric, the rest will be obvious.
    – Nobody
    Commented Sep 13, 2023 at 8:03
  • @Ecnerwal I reckon the size is going to be the horizontal size as it's easier to measure. Anyway the effect of trigonometry is far smaller than the variability in rainfall
    – Chris H
    Commented Sep 13, 2023 at 10:14
  • Is that US gallons or imperial gallons? (1 imperial gallon ≈ 1.2 US gallons) Commented Sep 13, 2023 at 14:57

3 Answers 3


Thats 4 X 8 X 1/12 cubic feet or 2.67 cubic feet.

1 cubic feet is 7.4805194805 gallons

So the answer is almost 19.95 or 20 gallons


The calculation for volume of a cube (and your roof+rainfall is a very squashed cube) is length x width x height. After converting length and width to inches you get 48 x 96 x 1 = 4608 cubic inches. Using this calculator, you get 19.945 gallons.

Your 4'x8' roof can capture 3" of rain in a 60 gallon system. A 12' x 8' roof would capture 1" of rain in that same system.

  • 1
    Or ; 231 cu in. / gallon. Commented Sep 12, 2023 at 14:56

First you compute the roof area, then you multiply by the amount of rain to have the volume of rain, and finally you convert to gallons:

        in          in                3             gallon
(4ft·4×3--)·(8ft·4×3--)·1in = 512×9 in = 4608 c_in -------- = 19.95 gallon
        ft          ft                             231 c_in

 side 1       side 2   rain
 ------- area ------- 

May I respectfully ask "Why 231?"?

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