How to Place and Size a Rain Garden to Fit Your Landscape

Installing a rain garden is a great way to reduce runoff and return water to its source.
Rain gardens are growing in popularity as an environmentally friendly way to infiltrate rainwater into your home landscape. This has great potential to reduce the overall demand on municipal sewerage systems and is a cost-effective means of effectively managing stormwater runoff on site.

There are several ways to create a rain garden. The first step is to decide where you want to place your rain garden and its size. In this series, we’ll look at the basics of rain garden design. You can use it to guide your DIY installation or to gain a more complete understanding of rain garden best practices.

What is a rain garden? A rain garden is a planting area designed to capture rainwater and storm runoff and slowly seep into the landscape. They use natural processes to slow, filter, and absorb stormwater, recharge groundwater, and contribute to the overall water quality of our communities.

Select a location

To make sure you’re choosing the best location for your rain garden, start with this checklist. Check out the following on the site:

✓ It should be at least 10 feet from the foundation of your home and all structures.
✓ Full or partial sun
✓ Downhill from the source of runoff (i.e. Downhill from the roof downspout if collecting
water from the roof).
✓ Flat or gently sloping areas
✓ In well-drained areas; Because rain gardens slowly return water to the soil, it is important that the native soil in your chosen location does not get wet.
✓ Not directly above utility lines or septic systems
✓ Suitable for overall landscaping in terms of circulation, views, entertainment areas,
children’s play areas and other garden beds. 

Determination of discharge source

Rain gardens can collect rainwater that runs off impermeable also called impervious surfaces. Sloped lawns experience surface runoff during severe storms, and rain gardens can also capture this. Leaks can be caused by:
✓ Rooftop
✓ Driveway and parking space
✓ Patios and terraces

Calculate the surface area of your source so you can determine how large you want your rain garden to be. Use your home’s square footage to calculate the roof area or the portion of the roof that drains into a rain garden, or measure your driveway or other paved surface to determine square footage.

Decide on the size of your rain garden

The size of your rain garden will depend on the amount of stormwater runoff you want to capture and the type of soil. Most residential rain gardens range from 100 to 300 square feet.

After calculating the surface area of your water source, gather the following information to determine the size of your rain garden.

Determine soil type: Sand, silt, or clay

Determine the slope of your yard: Flat (less than 4%); moderate (5-7%); steep (8-12%)
Use this handy calculator from the rain garden alliance to input the numbers for recommended size and depth. There are many calculators in the world. You can use this to get started, but we recommend consulting with a professional who can double-check your calculations and provide guidance.

Depth. A rain garden is not a swamp. Do not store or pool water for more than a few hours after a storm has passed. Instead, it should drain freely into a flat bottom that is 4 to 8 inches deeper than the surrounding ground.

The bottom of the rain garden should be level so that the collected water drains evenly. The steeper your yard’s existing slope, the deeper the bottom of your rain garden. Use the following depth ranges as a guide.

✓ Slope less than 4%: The depth should be 4 inches.
✓ 5-7% slope: The depth should be 6 inches.
✓ 8-12% slope: The depth should be 8 inches.

Length and width. Divide the total square footage by the length and width dimensions. Typically, rain gardens are twice as long as they are wide, with their longest side perpendicular to the downward flow of water. This allows the rain garden to efficiently capture as much water as possible while also allowing the water to spread across the bottom of the rain garden. However, you are not limited to rectangular shapes. Rain gardens can take on organic shapes, such as a crescent or kidney shape.