Determine Drainage Area
Area Draining to Garden
Sizing your rain garden
What's the right size for you?
Did you know that there's an easy way to find out? Just measure the square footage of the area where rain will runoff and divide this number by three. This will give you the right size rain garden to capture roughly 30% of the stormwater from this area.
Here's an example using a driveway:
- Multiply the driveway's square footage, 42' X 50', to find 2100 square feet of runoff area.
- Divide by 30%. Your rain garden should be 700 square feet.
- NOTE: The rain garden can be any shape, as long as it is on the downslope with a flat base and a depth of six to 12".
For rain gardens attached to downspouts, there are two additional steps. For example, take a 1,000 square-foot roof panel with two downspouts, one of which is connected to your rain garden. Divide the 1,000 square feet by two (the total number of downspouts on this panel). Multiply by one (the number of downspouts connected to your rain garden) and then divide by three to discover you should have a 167-square foot rain garden to treat the stormwater from the downspout channeled to your rain garden.
Most important, disconnect your downspouts from our sewer system and channel stormwater to rain gardens and rain barrels. Even if you don't have the space or resources for the "right-sized rain garden," remember that the smallest measures add up and help improve our area's management of stormwater. Remember, every drop matters, every drop counts!