Sizing a Rain Garden
Measure the area of your roof that uses the downspout you want to drain into the rain garden.
Measure only the footprint of the roof area; the same amount of rain falls on your roof whether it's flat or pitched.
If one gutter has two downspouts, assume half the water goes to each.
Divide the area by 3.
For example, suppose your downspout drains a 10' X 10' roof area, or 100 square feet. 100 divided by 3 is about 33 sq. ft, and that is the area your rain garden should be to hold water from a 1 inch rainfall.