How to determine the area draining to the garden:

The Goal: To determine the % of your roof that feeds your downspout.

Note: A rough estimate will do. Typically homes have four downspouts, each capturing 25% of the roof runoff.

1. Determine your drainage footprint. This is the square footage of your roof/lawn. For gardens less than 30ft from a building foundation that collect water from only the downspout, simply calculate your building's square footage. If you do not have this information, you may use a measuring tape to determine the length and width of your building. Then, multiply the length times the width to get your total square footage.
2. For gardens greater than 30ft from the building foundation, you must consider the lawn area in addition to the roof area. Measure the portion of the lawn that feeds the rain garden. For example, if you have 4 downspouts, visualize your building in 4 quadrants, each quadrant receiving water from a downspout. Then measure the length and width of the lawn area that is receiving water from only the downspout you are considering for the rain garden. Multiply the length and width together then add to your home square footage estimate.
3. Multiply the roof, and lawn areas if applicable, by the percent of the roof that feeds your down spout.
   Example:
   Drainage Area: 2,100 sq. ft. x .25 = 525 sq. ft.
   Your drainage area in this example would be 525 square feet