### Kate Venturini, BLA, MA Acknowledgement: Content adapted from

```Sizing
Kate Venturini, BLA, MA
Acknowledgement: Content adapted from Dr. Michael Dietz,
University of Connecticut
Rain Garden Sizing
Two options:
• Simple Method
– Sized to store 1 inch of runoff from 100%
impervious watersheds
• Soil-Based Method
– Multiply drainage area by soil sizing coefficient
based on DIY texture analysis
Rain Garden Sizing
Simple Method
• Calculate area of roof feeding to garden
30 ft
50 feet
Rain Garden Sizing
• Impervious surface:
50’ x 30’ = 1500 ft²
• Drainage area:
1500 ft² / 2 = 750 ft²
– This is because only half the
roof contributes to the garden
• Rain Garden area:
750 ft² / 6 = 125 ft²
– This just sizes the garden to hold
1 inch of water from the roof in
a 6 inch deep rain garden
30 ft
50 ft
Rain Garden Sizing
How do I lay out 125 ft²?
– Garden can be shaped
in a variety of ways
– Should be twice as long
as it is wide
– Should be oriented to
intercept as much
runoff as possible
9’
10’
14’
12.5’
Rain Garden Sizing
• In the East and
Midwest,
around 90% of
storms are 1”
or less
Rain Garden Sizing
Soil-Based Method
• Based on infiltration rates of soil texture groups and rain
garden depth
Rain Garden Surface Area in Sandy Soils (Sand, Loamy Sand and Sandy Loam)
(square feet)
Drainage Area
(Square feet)
200
400
600
800
1000
for 4 inch deep
garden
38
76
114
152
190
for 6 inch deep
garden
30
60
90
120
150
for 8 inch deep
garden
16
32
48
64
80
Rain Garden Sizing:
Example
Drainage area
Rain Garden Location
Drainage area is 100% impervious = 1,917 ft2
Rain Garden Sizing:
Example
Simple Method
• If watershed is 100% impervious, size to capture 1” of
runoff
• 1,917 ft2 / 6 = 320 square feet (6 inches deep)
Rain Garden Sizing:
Example
Soil-Based Method
• Drainage Area: 1,917 ft2
• Rain Garden Depth: 6”
• Soil Type: Sandy
Generally:
- 4” deep  multiply
drainage area by 0.19
- 6” deep  by 0.15
- 8” deep  by 0.08
Rain Garden Surface Area in Sandy Soils
(Sands, Loamy Sands and Sandy Loams)
(square feet)
Drainage
Area
for 4 inch for 6 inch for 8 inch
(Square
deep
deep
deep
feet)
garden
garden
garden
200
38
30
16
400
76
60
32
600
114
90
48
800
152
120
64
1000
190
150
80
1500
285
225
120
2000
380
300
160
Rain Garden Sizing:
Example
Soil-Based Method
• 1917 ft2 x 0.15 = 288 square feet *
• Why less surface area than Simple Sizing Method?
– Sandy soils = faster infiltration and less surface area
One plot side was not large enough for our required
rain garden area, so we oversized it!
12’9”
80”
14’3”
10’3”
177 sq ft
5’4”
80”
12’0”
29’6”
6’0”
230 sq ft
38’3”
Walkway
Walkway slope = 1%
toward storm drain
Rain Garden Sizing
Is it really that big of a deal?
1 inch of rain isn’t much, right?
Let’s see how Google sees the
world…
Rain Garden Sizing
• Impervious
Cover =
24,879 ft2
• 1” of rain=
15,509 gallons
stormwater
• Average year =
48” or 744,432
gallons!
```