To design your rain garden, we must first determine what size it should be.

The size of your rain garden will be determined by the combination of impervious area (roof, driveway, etc.) and previous area (lawn) that feed into your garden.


Note the downspouts you chose earlier. Measure the roof area that feeds into these downspouts.

To measure the area, consider using a tool like Durham GoMaps

Simply search for your address and use the Area Measurement tool to determine the square footage of your roof, lawn, and/or driveway that will be feeding into the garden.

Now that you have a square footage for your previous (lawn) and impervious (roof+driveway) surfaces, we are going to size the garden to handle a 1-inch rain storm – this will allow your garden to fully infiltrate 90% of the storms in a given year.

To size for a 1-inch storm, we use the 10-by-10 rule.

Take 10% of the impervious area you measured earlier, and add it to 1% of the pervious area.

A rain garden 10-inches deep at this new square footage can now handle a 1-inch rain storm!

For example, if your rain garden is capturing water from a 10’x30’ roof, a 12’x30’ driveway, and 20’x30’ of lawn you have:

660 sq ft of impervious surface

600 sq ft of previous surface

We then take 10% of the previous surface area (66 sq.ft.) and add that to 1% of the previous surface area (6 sq.ft.) for a total of 72 sq.ft.

​A rain garden with dimensions 7.2’x10’ dug 10 inches deep will now handle a 1-inch rain storm!

Now we know where your rain garden is going and how big it will be.

It’s time to move on to
Step 3: Installing your garden!