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How to Calculate the Size of Your Rain Garden
After you have contacted your local jurisdiction to find out if you need a permit to disconnect your downspouts or if there are other special requirements, the next step in creating your rain garden is to figure out how big it needs to be. The general rule of thumb is to calculate the square footage of your roof or other impervious surface being managed and multiply that by 10. This is called the 10 percent rule.
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How to Calculate the Size of Your Rain Garden |
Impervious Surface Area Causing Runoff |
Recommended Size (10%) of
Rain Garden |
Potential Dimensions of Rain Garden |
400 sq ft |
40 sq ft |
4x10, 5x8, 6x7 |
500 sq ft |
50 sq ft |
5x10, 6x8 |
600 sq ft |
60 sq ft |
4x15, 5x12, 6x10 |
700 sq ft |
70 sq ft |
5x14, 7x10 |
800 sq ft |
80 sq ft |
7x12, 8x10, 9x9 |
900 sq ft |
90 sq ft |
6x15, 7x13, 9x10 |
1000 sq ft |
100 sq ft |
7x15, 10x10 |
1250 sq ft |
125 sq ft |
8x16, 10x13 |
1500 sq ft |
150 sq ft |
10x15, 12x13 |
1750 sq ft |
175 sq ft |
9x20, 12x15 |
2000 sq ft |
200 sq ft |
10x20, 14x15 |
2500 sq ft |
250 sq ft |
10x25, 13x20, 5x17 |
The final step is to calculate if the rain garden can hold the volume of rain water coming from impervious surface over time. Many jurisdictions require what is called an infiltration or percolation test. It basically entails digging the depression for the rain garden and then filling it with water, and measuring how long it takes for the water to drain out.
As a rule of thumb, if the water drains at least two inches in an hour, your soil has adequate drainage for a rain garden. If not, you may need to amend the soil with a more porous material such as sand. The key to a successful rain garden is to minimize the amount of standing water in the depression. This will affect which plants you can use and the last thing you want to deal with is creating a mosquito breeding ground instead of a functional rain garden.
