# Mathematical formula for capturing amount of 1 inch rainfall on a tin roof messuring 4 ft x 8 ft?

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.

• 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. Commented Sep 12, 2023 at 15:12
• I think there are a few imperial units you haven't mentioned yet! :-) Commented Sep 13, 2023 at 0:12
• Switch to metric, the rest will be obvious. 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 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

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.

• 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?"?