[header] How to calculate roof runoff [title] Determine the footprint of your structure (f) in square feet : [substeps] Measure along the length of the house or building and along the width of the house or building (in feet). Multiply the two figures together to obtain the square footage of your roof. [title] Determine the percentage of water collected from each downspout (p). 
[substeps] Count the number of downspouts coming from your roof. Divide 100 by the number of downspouts in order to obtain the percentage of water collected from each downspout.