The size of your rain garden determines how much of the runoff on your property you are able to manage. To determine how big your rain garden should be, you have to figure out how big your drainage area is and consider factors like your soil type and how deep your rain garden will be.

Your rain garden should be no more than 4-8 inches deep. After all you aren’t creating a pond! If you are digging your garden into a slope, make sure that the planting area is level.

The type of soil you have in your garden determines how quickly water will drain. Obviously water drains the most slowly through clay soils and most quickly through sandy soils. Amend your soils if necessary to balance drainage and retention.

The hardest part about determining how big your rain garden should be is to calculate the size of the area that will drain into your garden. For example if your garden will be sustained by runoff from your roof, you need to calculate the square footage of your roof.

To find the square footage or your roof, multiply your house’s width by its length. For example, if your house is 40 feet × 40, your roof is 1,600 square feet.

When you calculate the square footage of your roof, don’t forget that you may have more than one downspout. A typical house has four, so a rain garden placed beside the downspout on the south side of the back yard might be fed only 25% of the water from the rooftop. So given the previous example you would divide 1,600 square feet by 4, giving you a total of 400 square feet.

If you are placing your rain garden more than 30 feet from a downspout, take the yard space into account when you figure the drainage area. Measure the distance from the house and the width of the drainage area and add it to your roof figure and find the soil type for your garden. Multiply your size factor by the drainage area to get your total garden size.

If you don’t have the money or the space to make a garden as large as the sizing calculations suggest, you can still manage a lot of your runoff with a smaller rain garden.

Let the way the water drains help you determine your garden’s shape. For example, your garden length should be perpendicular to the water source, such as a downspout. You want the water to spread evenly throughout your rain garden and not pool in one area.