Rain Garden Design Series: Part Two, Simple Design Calculations and Plant Selection

In our last Watershed Landscape post, Rain Garden Series Part One, we explored the first steps to building a rain garden, which starts with good planning. This included understanding stormwater runoff on your property, site selection criteria, and conducting simple tests to know more about your soils and how well your drainage is.

Today’s post will cover the next steps: Design Calculations, simple formulas to find out the size of your rain garden, how deep it needs to be and we will discover which plants do well in the rain garden!


Tools/Skills Needed: Stakes, tape measure, line level and string to determine slope. Graph paper to plan for plants and dimensions.

Costs: $10-15 if you do not have any of the above tools

Time: 1-2 hours

Level of Difficulty: Easy


What Size Should A Rain Garden Be?

The idea of a rain garden is to capture some or most of the rainfall shedding from your rooftop. Rain gardens do best when they are 1/3 of your drainage area, but can be larger or smaller. To calculate what the ideal size of your rain garden should be, you will first need to know the impervious area draining to your site. Calculate length x width to get the square footage of half your roof.

Check out this example of our rain garden at Cave Springs Community Building to complete this step.

Cave Springs Rain Garden Drainage

If half of your roof is 800 square feet, you will divide by the number of downspouts to determine your drainage area.


Ex.) 800 sq. ft./ 2 gutters = 400 sq. ft of drainage to each gutter.

A typical rain garden will be 10-30% of your drainage area, so multiply by .3 to get the size of your rain garden:

Ex.) 400 sq. ft. drainage x .3 = 120 sq. ft. rain garden needed to capture 30% of rainfall.


How Deep Should A Rain Garden Be?

Rain gardens are depressions in the landscape that capture and allow water to infiltrate into the ground. To determine how deep your rain garden needs to be, or the rainfall capacity, you will need to know the slope of your property. Slope is easily calculated with a tape measure, stakes, string and a level. Find the directions to determine slope here.

Slope Measurement

Once your slope is determined, follow our guideline to know the rainfall capacity of your rain garden. Prairie Grove EAST student shown measuring slope. 

Slope is less than 4% = 4” Deep Rain Garden

Slope is 5-7% = 6” Deep

Slope is 8-12% = 8” Deep

Slope is more than 12% = Select Different Location


How to Calculate the Amount of Materials Needed

If you conducted a simple soil test, discussed in Part One, you may have found that you have clay soils or soils that have clay in them. If you would like to improve your soil’s drainage, you can amend the soils. The recommended soil mixture should be 1/3 Topsoil, 1/3 Bank-run Sand, 1/3 Compost. We recommend 6-8” of soil amendments for your rain garden.

Soil Amendments

Soil Amendments can be calculated by following this formula.


Any easy guideline for calculating mulch is for every 100 sq. ft. of garden, you will need 1 cubic yard of mulch.

Ex) 120 sq. ft. rain garden would need 1 yard of mulch.


What Plants Are Best in a Rain Garden?

Native plants perform excellent in a rain garden. They work hard to stabilize soil, uptake nutrients, mitigate stormwater, they provide interest throughout the year and are beneficial to our local wildlife. A few simple tips for your rain garden plant design include:

–       Learn which plant does best in moist, medium and dry areas.

–       Layer shrubs, plants, and grasses to add dimension and depth to the design.

–       Keep the plant palette simple and mass together like plants.


Learn more about Native Plants by referencing our Native Plant Brochure.

Review our Rain Garden Planting Plans.

You can use graph paper to draw out and play with the different shapes and plant materials of your rain garden. This will help you prepare for the next steps of installation, which will be covered in Rain Garden Series, Part Three: Installation.


Spring Rain Garden Academy

The Spring Rain Garden Academy will provide both classroom and hands-on training on how to design and install a rain garden. Participants will get to help plant one of five rain gardens during the training. Registration is required, sign up today!


Related Posts:

Watershed Landscape: Rain Garden Design — Part I

Watershed Landscape: Rain Garden Pre-Spring To-Dos

Watershed Landscape: Rain Gardens, A Blooming Good Idea!