Calculators / Rainwater and drainage
How big should your rain garden be?
Tell us how much roof drains to the garden, your soil and how deep you will dig. You get the garden size that soaks up the runoff from a typical storm.
Updated · How we check our numbers
Build it this size
125 sq ft
A rain garden of 125 sq ft, say 8 by 16 ft, holds the runoff from 500 sq ft of roof in silty soil.
- Garden size
- 125 sq ft
- Size factor
- 0.25
- Depth
- 6 to 7 in
- Soil drains
- 0.5 in/hrsilty
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Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.
Not enough room? Try a dry wellSame roof, underground
size factor = silty soil, 6 to 7 in deep, under 30 ft from the downspout = 0.25
garden = 500 sq ft × 0.25 = 125 sq ft
for example 8 ft × 16 ft, or any shape with the same area
At least 10 ft from the house, on ground sloping less than 12%, never over a septic system, and not where water already sits after rain.
Dig a hole 8 in deep, fill it to 6 in, and time how long it takes to drain. 6 in in 12 hours is 0.5 in an hour. Slower than that, try another spot.
Dig the bottom level so water spreads across the whole garden. Use the soil you dig out to build a low berm on the downhill side.
A rain garden should be dry within five days of rain. Mosquitoes need 7 to 12 days of standing water to hatch.
Call 811 a few days before you dig so buried lines get marked. It's free.
How we work it out
We use the size factors from University of Wisconsin-Extension and the Wisconsin DNR. Multiply the roof area draining to the garden by the factor for your soil, depth and distance:
garden sq ft = drainage area × size factor
| Soil | 3 to 5 in deep | 6 to 7 in | 8 in | 30 ft or more from the downspout |
|---|---|---|---|---|
| Sandy | 0.19 | 0.15 | 0.08 | 0.03 |
| Silty | 0.34 | 0.25 | 0.16 | 0.06 |
| Clayey | 0.43 | 0.32 | 0.20 | 0.10 |
Their example: a 100 by 20 ft house with four downspouts sends 500 sq ft to each one. In silty soil, 6 to 7 in deep, that's 500 × 0.25 = 125 sq ft of garden.
Test the soil. Oregon State Extension's test: dig a hole 8 in deep, put a pencil 6 in above the bottom, fill to the pencil and time how long it takes to drain. 6 in in 12 hours is 0.5 in an hour. Don't test within a day of a half-inch rain or on frozen ground.
Where. At least 10 ft from the foundation, on ground sloping less than 12%, not over a septic system, and not where water already stands. It should drain within five days.
Where the numbers come from
Also
Rainwater and drainage
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