STEP 1: Roof Size and Downspouts

Your roof is one of the biggest sources of water runoff. By redirecting one or more of your downspouts, you can reclaim that water and put it to good use... your beautiful rain garden. If you don’t have a roof, skip to the next step.

  • Let’s start by estimating your total roof size.
  • Now tell us how many downspouts you have on the side of the house you want to put your rain garden
  • Lastly, how many of those downspouts you want to tie into your rain garden? We recommend at least one.

My Roof Size is Approximately: (Don’t have a roof, got to “next step”)

I have and

STEP 2: Estimate Drainage Area and Yard Composition

We’ve tackled the roof, now let’s look at your yard. Features such as patios and landscaping, and yes, even grass, each contribute to runoff, just at different rates. To be sure your rain garden is large enough to handle the amount of water it will receive, let’s estimate the surface types in the portion of your yard that that drains toward it.

  • Look at your yard and then consider the portion of it that will drain towards your rain garden. Estimate the square footage of that portion.
  • Look at that same drainage area. Estimate the % of each surface type. This is your yard composition.
  • Then select “go” to see the results

Approximate size of my yard that drains towards my rain garden:


The composition of that area is as follows:

* click a surface type below to include it in your yard composition.
Use the sliders to adjust percentages.

RESULTS: Rain Garden Size and Cost Estimate

Based on the information you provided, during a 1-inch rainfall event, your roof and property will shed:

gallons of water

To capture 100% of this runoff, your rain garden should be approximately 13 x 13 feet and 8 inches deep:

Adjust Depth

4 in. 6 in. 8 in. 10 in. 12 in.

Your estimated cost for the above figures would be:

Estimate based on avg. $2.75 / sq.ft.