Rain Garden Size Requirement Instructions

The size needed is determined by the area of the surface to be drained (watershed) and the type of soil present at the garden site (sand, loam, clay).
1. Measure the length and width of the roof, road, etc. Calculate the area in square feet.
2. Determine the type of soil present. Follow the instructions on the Soil Texture Worksheet.
3. Decide on the depth of your garden. A typical rain garden is between four and eight inches deep.
4. Using Chart I below, multiply the watershed area by the size multiplier given on the chart for the desired depth. Example: Your roof area is 50' by 15'=750 sq. ft. Your garden is 6-7 inches deep, and you have loam soil. Multiply 750 sq. ft. x 0.25 (the size multiplier for your soil type) to equal 187.5 sq. ft. of rain garden space required. Round up to 200 sq. ft.

Chart II shows the size of the rain garden required to absorb 90 percent of the water as a percentage of the roof area. This figure is determined by the type of soil and how fast the water is absorbed. Example: To absorb 90 percent of the runoff requires a garden 90 sq. ft. in area (or 1/5, 20 percent the size of the roof) at a rate of 0.4 inches/hour.