There are some easy calculations that can be made to determine the volume of water that will run off of your roof in a typical storm. This will give you an idea of what the storage capacity of your rain garden or rain barrel should be.

Enter the following data and click on the "Compute" button to find the volume of water that will run off your roof in a typical storm and the volume that will run off over a year.

Footprint of House: Width ft x Length ft
Number of Downspouts
Inches of Rain inches

        

The number of gallons from each rainspout for inches of rain is gallons.
The number of gallons from each rainspout over a year is gallons !!

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These numbers are computed as follows:

The footprint of my house (in feet) is ft x ft = ft². This is the area of my roof in square feet.

I have downspouts coming from my roof. Therefore, I estimate that each of these downspouts collects water from % of my roof (i.e. if you have 4 downspouts, you can assume each of them collects 25% of your total roof runoff.

The area in square footage that each downspout collects water from during a storm is computed by multiplying the area of your roof in square feet (step 1) x the percentage (as a decimal) from step 2 = . We'll call it A_d.

Now we will calculate the volume of water that each downspout of your roof collects in a rain event. We will use the following calculation:

A_d x R(rainfall in inches) x C (conversion factor) = V (volume in gal)

The conversion factor is the number of gallons per ft² of rain that fall in a 1 inch rainstorm. It will always be 0.62.

So, for example.

If we have an A_d of 300 ft², the volume of water which would come out of each downspout during a 0.5 inch rainstorm would be roughly:

V = 300 (Ad) x 0.5 (R) x 0.62 (C) = 93 gallons

For your home the volume is:

A_d x R x C = V gallons from each downspout.

Now calculate the runoff coming out of each downspout every year! Northern Vermont has an average of 35" of rain each year.

A_d x R x C = V gallons from each downspout!!