How Big Should a Rain Garden Be? Sizing Math With Worked Examples
A rain garden is usually 1/8 to 1/3 the size of the roof and paving that drain into it, and the exact size depends on your soil and slope. Here is the math, with worked examples for sand, silt and clay.
A rain garden should usually be 1/8 to 1/3 the size of the roof and paved area that drains into it, and where you land in that range depends on how fast your soil soaks up water. Sandy soil gets by with a small garden, while a shallow garden on clay can need closer to half the size of the area feeding it.
Quick answer
Measure the roof and paving that drain to the spot, then multiply by a size factor: about 0.15 to 0.19 for sand, 0.25 to 0.34 for silt and 0.32 to 0.43 for clay at the usual 3 to 7 inch depth. A 1,000 square foot drainage area on silty soil needs about 250 square feet of garden, such as 25 by 10 feet, 6 inches deep and at least 10 feet from the house.
Two ways to size a rain garden
Both methods start with the area that drains to the garden and size it for a typical 1-inch rain. The Massachusetts Office of Coastal Zone Management rain garden fact sheet multiplies that area by a size factor for your soil and depth; slower soil gets a bigger factor. Penn State Extension multiplies it by 0.083 (1 inch in feet) to get cubic feet of runoff, then divides by the ponding depth in feet. Penn State’s own rule of thumb is 10 to 20 percent of the hard surface draining in, and it says that figure varies a lot with soil and depth.
Step 1: Measure the drainage area
Count every surface that sends water to the garden: the part of the roof feeding that downspout, plus any driveway, walk or patio that slopes toward it. Massachusetts also counts lawn on compacted soil, which sheds water much like pavement.
Measure the roof the way it sits on a site plan. On a 40 by 30 foot house where half the roof drains to one corner, that’s 40 × 30 × 0.5 = 600 square feet. Massachusetts adds 400 square feet of driveway, walk and lawn in its example, for 1,000 square feet. One inch of rain on that is about 83 cubic feet of water, roughly 620 gallons at 7.48 gallons per cubic foot.
Step 2: Set the depth from your slope
On sloped ground you dig into the uphill side and use that soil for a low berm downhill, so the bottom ends up level. To measure slope, set a stake at each end of the garden, run a level string from the base of the uphill stake, and measure up to the string at the downhill stake. A 6-inch rise over 120 inches is 6 ÷ 120 × 100 = 5 percent.
| Ground slope | Rain garden depth |
|---|---|
| Under 4% | 3 to 5 in. |
| 5 to 7% | 6 to 7 in. |
| 8 to 12% | 8 in. or more |
Penn State calls slopes steeper than 10 percent not ideal, and Massachusetts keeps any berm to 6 to 8 inches high. Don’t go deeper than you need: Massachusetts notes that deeper water filters less well, and it wants standing water gone within 24 to 36 hours so mosquitoes can’t breed.
Step 3: Find your soil type
Penn State gives rough soaking rates of about 2.5 inches per hour for sand, half an inch for silt and a third of an inch for clay. Massachusetts identifies soil with a ribbon test: dig below the topsoil, roll a handful into a ball and squeeze it.
- Ribbon longer than 1.5 inches: clay
- Weak ribbon that breaks at about 1.5 inches or less: silt
- No ribbon: sand
Then test drainage. Penn State digs several holes 1 foot deep and 1 foot wide, fills them with water and times them; if they take more than 24 hours to drain, pick another spot. If water seeps into an empty hole within an hour, Massachusetts says the site doesn’t drain well enough.
Step 4: Multiply by the size factor
Garden area equals drainage area times the size factor. Pick a length and width that add up to at least that area.
| Soil | 3 to 5 in. deep | 6 to 7 in. deep | 8 in. or more |
|---|---|---|---|
| Sand | 0.19 | 0.15 | 0.08 |
| Silt | 0.34 | 0.25 | 0.16 |
| Clay | 0.43 | 0.32 | 0.20 |
Worked example 1: silt on a 5 percent slope
This is the Massachusetts example: 1,000 square feet of drainage area on a 5 percent slope, so 6 to 7 inches deep with a silt factor of 0.25. That’s 1,000 × 0.25 = 250 square feet. A 25 by 10 foot garden, 6 inches deep, holds 250 × 0.5 = 125 cubic feet, more than the roughly 83 cubic feet a 1-inch rain sends it.
Worked example 2: the same yard on clay
Same area and slope, but the ribbon test says clay. The factor rises to 0.32, so 1,000 × 0.32 = 320 square feet: 32 by 10 feet, or 20 by 16. On nearly flat ground the depth drops to 3 to 5 inches and the factor climbs to 0.43, for 430 square feet.
Worked example 3: one downspout, both methods
Penn State’s example is a 50 by 12.5 foot roof section, 625 square feet. One inch of rain on it is 625 × 0.083 = 52 cubic feet, and at 6 inches (0.5 foot) of ponding, 52 ÷ 0.5 = 104 square feet of garden.
The size factors at 6 to 7 inches deep give 625 × 0.15 = 94 square feet on sand, 156 on silt and 200 on clay. The volume method gives one answer for every soil; the factors grow the garden as the soil slows down. On clay, the larger answer is safer.
How deep should a rain garden be in clay?
Stay at the shallow end. At Penn State’s rate of a third of an inch per hour, 6 inches of water takes about 18 hours to soak into clay, and 8 inches takes 24, right at the limit. The same 6 inches takes about 12 hours in silt and under 3 in sand.
Dig the basin 3 to 8 inches below the surrounding ground. Massachusetts blends the soil to 50 to 60 percent sand, 20 to 30 percent topsoil and 20 to 30 percent shredded bark mulch, and skips compost because of its high phosphorus. In clay or silt, it digs another 6 to 18 inches, lays gravel or crushed stone, covers it with that mix and retests.
If a test hole still holds water past 24 hours, don’t force it. Massachusetts says rain gardens may not suit poorly drained or clay soils, ledge or a high water table; move the garden or use another fix from our guide to drainage solutions for clay soil.
Plants for clay: the bottom stays wet longest, so put plants that handle wet roots in the center and at the inlet, and drier-tolerant ones on the edges and berm. The Massachusetts wet list includes blue flag iris, swamp milkweed, cardinal flower and tussock sedge; its edge list includes butterfly milkweed, little bluestem, switchgrass and New England aster. Check that each is native to your region, or see our Pennsylvania native flowers guide.
Where to put a rain garden
- At least 10 feet from the foundation, per both Penn State and Massachusetts, so the water doesn’t end up in the basement.
- At least 50 feet from a septic system or well, per Massachusetts. Penn State says not to build near a septic tank, leach field, well, tree roots or buried utility lines.
- Downhill from the downspouts and paving, on fairly flat ground, but not in the low spot that already holds water. Just uphill of a soggy spot can be ideal.
- Bottom at least 2 feet above the water table. Check in spring, when groundwater is normally highest.
- Where overflow stays on your property, away from the road and the neighbors.
Moving roof water there usually means an extension or buried pipe; see how to bury a downspout. If a wet spot is why you’re here, read why your yard holds water after it rains first. Call 811 before you dig: Massachusetts says at least 3 days ahead, and Penn State notes it’s free to homeowners. Outside those states, our 811 dig laws guide has your state’s notice rule.
Inlet, overflow and upkeep
Penn State describes three working parts: a rock-lined inlet to slow incoming water, a low berm with gradual slopes, and an erosion-resistant overflow outlet, often lined with rock.
If the garden overflows in ordinary rains, Massachusetts suggests enlarging it, adding a second garden or sending overflow down a swale; our swale vs French drain comparison explains how a swale carries water. Inspect every 3 to 6 months and after big storms, and scrape out silt where water enters.
Sending the overflow down a swale? Our guide to how to build a drainage swale covers the fall, width and outlet. For the downspout feeding the garden, see how far downspouts should extend, and for every other wet-yard fix, the yard drainage guide.
Builder’s note: Lay the garden out with its long side across the slope, along the contour. The berm stays shorter, the uphill cut stays shallower, and the bottom is easier to get level. Check it with a string line before you plant: a bottom that tips sends all the water to one end.
DIY or hire a pro
A garden of a couple hundred square feet on open lawn, fed by a downspout extension and with a passing drainage test, is a fair weekend project.
Bring in help when the garden needs a machine (Massachusetts warns heavy equipment can compact the soil), water has to cross under a driveway or walk, clay fails the drainage test, or the site is near a septic system, well, steep slope or property line. Penn State also recommends checking local stormwater rules and plant height ordinances first. If you’d like the location, size and overflow worked out on paper, our backyard drainage plans are drawn from your photos and measurements.
Frequently Asked Questions
Is there a rain garden size calculator?
You can do it on a phone. Multiply the drainage area in square feet by a size factor (0.08 to 0.43, by soil and depth), or multiply it by 0.083 and divide by the ponding depth in feet.
How deep should a rain garden be?
Plan for 3 to 8 inches of ponding: 3 to 5 inches on slopes under 4 percent, 6 to 7 on 5 to 7 percent and 8 or more on 8 to 12 percent. Stay shallow in clay.
Can you put a rain garden in clay soil?
Yes, if a test hole drains within 24 hours. Use the clay size factors (0.32 to 0.43), keep it shallow and consider rebuilding the bottom with gravel and a sandy soil mix.
How far from the house should a rain garden be?
At least 10 feet from the foundation, according to both Penn State Extension and Massachusetts CZM, and at least 50 feet from septic systems and wells.
Sources: Massachusetts Office of Coastal Zone Management, Stormwater Solutions for Homeowners Fact Sheet: Rain Gardens; Penn State Extension, How to Make a Rain Garden (video and transcript)
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Written and reviewed by Thayne Muthler, owner of Muthler Landscaping & Construction (PA HIC #PA212584). How we check facts: editorial policy.