• The depth that your test hole has drained will be the depth of your rain garden.
  • Based on the depth of your garden, you will need to determine the length and width.
  • Calculate how much drainage area will be directed into the garden (for example, square footage of the amount of rooftop draining into the downspout you will disconnect).
  • Divide your drainage area by your depth.

Example: I have 240 sqft. of rooftop draining to a downspout I will disconnect. My test hole drained 4” in 24 hours. My rain garen will be 4″ deep. I then divide 240 by 4, and my rain garden will be 60 sqft.

(This calculation is from The Blue Thumb Guide to Raingardens by Rusty Schmidt, Dan Shaw, and David Dods, 2007.)