How big should a rain garden be?

Multiply the drainage area by a size factor from the University of Wisconsin-Extension: 0.19 to 0.43 for a garden 3 to 5 in deep within 30 ft of the downspout, 0.08 to 0.20 for 8 in deep, depending on the soil. For 300 sq ft of roof on silty soil, 6 to 7 in deep, that is 75 sq ft; with sandy soil 8 in deep it would be 24 sq ft, and with clay 3 to 5 in deep 129 sq ft.

Why does soil change the size?

Water soaks into sand quickly and into clay slowly, so a clay garden has to be larger to hold the same runoff while it drains. Take a 600 sq ft roof at 6 to 7 in deep: 90 sq ft on sand, 150 sq ft on silt and 192 sq ft on clay. Soil that will not drain at all is a reason to pick another spot, not to build a bigger garden.

What if the garden is more than 30 feet from the downspout?

The Wisconsin tables use a different, much smaller factor: 0.03 for sandy soil, 0.06 for silty and 0.10 for clayey, for any depth from 3 to 8 in. A 600 sq ft drainage area on silt then needs 36 sq ft instead of 150 sq ft (6 to 7 in deep, within 30 ft). Pick the distance in the calculator. A long pipe or swale from the downspout is a way to get the water there.

How do I find the drainage area?

Measure the footprint of the roof section, or the paving, that sends water to the garden, as you would on a plan seen from above: length × width, including the overhang. Do not use the sloped area. If a gutter sends only part of the roof to one downspout, measure just that part. Take off any section that drains somewhere else with the “Add a part that drains elsewhere” button.

How many gallons does a rain garden hold?

Ponded volume = garden area × depth ÷ 12 × 7.48052 gallons per cubic foot. The 75 sq ft example ponds 327 gallons at 7 in. That is more than the 178 gallons that 1 in of rain sends off 300 sq ft of roof (300 × 1 ÷ 12 × 7.48052 × 0.95), because the garden also drains into the soil during a storm and the size factors cover bigger storms; the garden holds the runoff from about 1.8 in of rain before it overflows, ignoring the water that soaks in.

How much water runs off a roof in a 1 inch rain?

Area × 1 in ÷ 12 ft × 7.48052 gallons per cubic foot × the runoff coefficient. For a roof at 0.95, 1,000 sq ft gives 592 gallons; with nothing lost it is 623 gallons. A roof is close to a coefficient of 1 because little soaks in, and the calculator uses 0.95 as an estimate; use the 1.00 option if you want the worst case.

How do I test whether my soil drains well enough?

Dig a test hole where the garden will go (6 to 8 in deep is typical), fill it with water to soak the soil, then fill it again and see how long it takes to empty. Guidance varies: the Missouri Botanical Garden says it should empty within 24 hours (a rate of at least 0.25 in per hour), and the DC RiverSmart Homes worksheet allows up to 36 hours. Water standing longer than that means the spot does not drain well enough; try another spot. Run the test where you will dig, and at a time when the soil is not already wet or frozen. Your local extension office or stormwater program may set a different limit.

Can I use this for a driveway or a patio?

The Wisconsin manual is written for small roof areas, and says not to use it for parking lots, busy streets and other heavily used pavement without pretreatment. A driveway or patio can be entered here, using a coefficient of 0.90 for the runoff volume, but the size factor is the same as for a roof; treat the result as a rough starting point. For 500 sq ft of paving (25 × 20 ft) the garden is 125 sq ft and 1 in of rain gives 281 gallons. Oil, salt and silt in the runoff are why pavement needs more care.

How much compost and mulch does a rain garden need?

Garden area × the layer depth ÷ 12 ÷ 27 cubic yards. The 75 sq ft example needs 0.69 cu yd of compost mixed in at 3 in (25 bags of 0.75 cu ft) and 0.69 cu yd of mulch at 3 in (10 bags of 2 cu ft). The 2 to 4 in of compost and 2 to 3 in of mulch are typical figures, not rules; sandy soil may need little, and clay needs advice on grit and compost from your extension office. Use shredded hardwood mulch, which knits together and washes out less than chips.

What depth should I pick?

Deeper gardens are smaller: silty soil on 300 sq ft needs 102 sq ft at 3 to 5 in, 75 sq ft at 6 to 7 in and 48 sq ft at 8 in. The Wisconsin method covers depths from 3 to 8 in. Shallower is easier to dig and to keep safe for children and pets; deeper takes less room but holds more water for longer. Steep sites need more digging and a berm.

Rain Garden Size Calculator: How Big Should the Garden Be?

A 20 × 15 ft piece of roof (300 sq ft of drainage area, about what one downspout drains on a small house) needs a 75 sq ft rain garden in silty soil if it is 6 to 7 in deep and within 30 ft of the downspout: 300 sq ft × 0.25 size factor = 75 sq ft. It ponds 327 gallons at 7 in deep, enough for the runoff of about 1.8 in of rain (1 in of rain gives 178 gallons off a roof). Plan on 0.69 cu yd of compost and 0.69 cu yd of mulch. Garden area = drainage area × size factor (University of Wisconsin-Extension); the factor is bigger for slow soil and shallow gardens.

Figures based on the sources listed belowUpdated 2026-10-18How we calculate

Drainage area length

Drainage area width

Soil typeSlower soil needs a bigger garden. If a water-filled test hole has not drained in a day or so, the spot is unsuitable (see the percolation note below), whatever the size.

Garden depthHow deep the water ponds. The size factor is written for the deepest depth in each band, so the ponded volume here uses 5, 7 or 8 in.

Distance from the downspoutThe manual uses a much smaller factor (0.03, 0.06 or 0.10 by soil, for any depth) when the garden is more than 30 ft from the downspout.

Surface that drains to itOnly changes the runoff volume, not the garden size. The Wisconsin method is written for small roof areas; paving is an extension of it, and busy pavement needs pretreatment first.

Rainfall depth

The storm to work the runoff for. 1 in is the usual yardstick; the garden size does not depend on it.

Compost layer to mix in (in)

Compost or other amendment spread over the garden and mixed into the soil. 2 to 4 in is typical; an estimate, so follow your local extension office.

Mulch depth (in)

2 to 3 in of shredded hardwood mulch is typical. Triple-shredded stays put better than chips.

75 sq ft rain garden

Drainage area counted: 300 sq ft
Size factor (garden area ÷ drainage area): 0.25, for a garden 7 in deep
Ponded volume (garden area × depth): 327.3 gal
Runoff from the rain entered: 177.7 gal
Rain it holds before it overflows (estimate): about 1.8 in of rain, before any water soaks in
Soil amendment (compost) to mix in: 0.69 cu yd (or 25 bags of 0.75 cu ft)
Mulch on top: 0.69 cu yd (or 10 bags of 2 cu ft)

Shopping list

Shredded hardwood mulch (2 cu ft bag)
Compost (bagged)
Round-point shovel
Garden wheelbarrow
Bow rake
Native rain garden plant seed mix

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Size factors by soil and garden depth

Garden area as a share of the drainage area (University of Wisconsin-Extension / Wisconsin DNR). The last column is for a garden more than 30 ft from the downspout, for any depth from 3 to 8 in.
Soil	3 to 5 in deep	6 to 7 in deep	8 in deep	More than 30 ft away
Sand (sandy)	0.19	0.15	0.08	0.03
Silt (silty)	0.34	0.25	0.16	0.06
Clay (clayey)	0.43	0.32	0.20	0.10

Rain garden size by drainage area, silty soil

Garden area within 30 ft of the downspout, silty soil, by the depth of the garden (factors 0.34, 0.25, 0.16). Worked by this calculator at build time.
Drainage area	3 to 5 in deep	6 to 7 in deep	8 in deep
200 sq ft	68 sq ft	50 sq ft	32 sq ft
400 sq ft	136 sq ft	100 sq ft	64 sq ft
600 sq ft	204 sq ft	150 sq ft	96 sq ft
1,000 sq ft	340 sq ft	250 sq ft	160 sq ft
1,500 sq ft	510 sq ft	375 sq ft	240 sq ft
2,000 sq ft	680 sq ft	500 sq ft	320 sq ft

Runoff and ponded volume by drainage area

For a roof (coefficient 0.95) and a silty-soil garden 6 to 7 in deep (ponding 7 in), within 30 ft. “Raw” is the rain with nothing lost (0.6234 gal per sq ft per inch). The last column is the rain the pond holds, before any soaking in.
Drainage area	Rain on it, 1 in (raw)	Roof runoff, 1 in	Ponded volume	Rain it holds
200 sq ft	125 gal	118 gal	218 gal	1.8 in
400 sq ft	249 gal	237 gal	436 gal	1.8 in
600 sq ft	374 gal	355 gal	655 gal	1.8 in
1,000 sq ft	623 gal	592 gal	1,091 gal	1.8 in
1,500 sq ft	935 gal	888 gal	1,636 gal	1.8 in
2,000 sq ft	1,247 gal	1,184 gal	2,182 gal	1.8 in

Methodology & assumptions

The size comes from the size-factor method in the University of Wisconsin-Extension and Wisconsin DNR manual Rain Gardens: A How-To Manual for Homeowners (2003). Garden area = drainage area × size factor. For a garden within 30 ft of the downspout the factor depends on the soil and the depth of the garden: sandy soil 0.19, 0.15 and 0.08 for 3 to 5 in, 6 to 7 in and 8 in; silty soil 0.34, 0.25 and 0.16; clayey soil 0.43, 0.32 and 0.20. More than 30 ft from the downspout the factor is 0.03 (sandy), 0.06 (silty) or 0.10 (clayey) for any depth from 3 to 8 in. The drainage area is the footprint of the roof or paving that sends water to the garden, length × width, with the “Add a part that drains elsewhere” button to take off sections that drain away. Use the flat footprint of a roof, not the sloped area. The manual is written for small roof areas: it says not to use it for parking lots, busy streets and other heavily used pavement, and a paved driveway or patio in this calculator is an extension of the method that you should check locally. The ponded volume is the garden area × the depth ÷ 12 × 7.48052 gallons per cubic foot (1,728 cu in ÷ 231 cu in per US gallon). The factors are written for the deepest depth in each band, so the volume uses 5 in for “3 to 5 in”, 7 in for “6 to 7 in” and 8 in for “8 in”; a shallower garden holds less. The runoff volume is drainage area × inches of rain ÷ 12 × 7.48052 × a runoff coefficient. A coefficient is the share of the rain that runs off a surface; a roof is close to 1, and this calculator uses 0.95 for a roof and 0.90 for pavement as typical planning estimates (published values vary with the roof material, the paving and the storm), or 1.00 if you want all of it. The “rain it holds” line divides the ponded volume by the runoff from 1 in of rain, so it shows how much more than 1 in the garden can take; it is an estimate that leaves out the water that soaks into the soil during the storm, which is why the size factors give a pond bigger than one inch of rain. The manual’s own sizing is meant to catch most of the runoff in a typical year, not every storm. The compost and mulch are the garden area × the layer depth ÷ 12 ÷ 27 cubic yards, with bags counted at 0.75 cu ft (the topsoil calculator’s standard bag) and 2 cu ft (the mulch calculator’s bag). Layer depths of 2 to 4 in of compost and 2 to 3 in of mulch are typical; they are estimates, not a standard. The calculator does not test your soil: dig a test hole and check the percolation as set out below, and keep the garden away from the foundation, septic fields and utilities (call 811 before you dig). Local rules and your county extension office have the last word.

University of Wisconsin-Extension and Wisconsin DNR, Rain Gardens: A How-To Manual for Homeowners (2003), and the UW-Extension handout “How to Build a Rain Garden” (size factors by soil and depth, and the 0.03 / 0.06 / 0.10 factors for gardens more than 30 ft from the downspout; the figures were read from search-result excerpts of the handout, so check them against the manual)
University of Wisconsin-Extension, Southeast Wisconsin rain gardens site, “Step 1: Sizing and Siting the Rain Garden” (search result only; page not opened)
Wisconsin DNR and Wisconsin Standards Oversight Council, Rain Gardens: A Guide for Homeowners and Landscapers (2018), which refers to DNR Conservation Practice Standard 1009 for complete guidelines (not used for the factors here)
Missouri Botanical Garden, “Is a rain garden right for your site?” (percolation test: the hole should drain within 24 hours, a rate of at least 0.25 in per hour; read from a search excerpt)
DC Department of Energy and Environment, RiverSmart Homes percolation test worksheet (a 36-hour limit and 0.3 in per hour; read from a search excerpt)
Missouri Botanical Garden, rainscaping guide, planting a rain garden: 2 to 3 in of shredded hardwood mulch (search excerpt only). Compost depth of 2 to 4 in is from the City of Atlanta’s rain garden guidance (search excerpt only, not opened)
NIST, Guide for the Use of the International System of Units (SI), appendix B.8: the US liquid gallon is 231 cubic inches
Runoff coefficients of 0.95 (roof) and 0.90 (pavement) are typical planning estimates, not published figures from the manual; secondary sources give ranges

How to size a rain garden

Measure the drainage area. Take the length and width of the roof section or paving that drains to the garden, as seen from above. Take off any part that drains elsewhere.
Test the soil. Dig a test hole where the garden will go, fill it with water and check how long it takes to drain. If it takes longer than about 24 to 36 hours, choose another spot.
Pick the soil and the depth. Choose sand, silt or clay, and a garden depth of 3 to 5 in, 6 to 7 in or 8 in. Choose the distance from the downspout: 30 ft or less, or more than 30 ft.
Multiply by the size factor. Garden area = drainage area × size factor. For example 300 sq ft × 0.25 for silt, 6 to 7 in deep, is 75 sq ft.
Check the volume and buy the materials. Ponded volume = garden area × depth ÷ 12 × 7.48052 gallons. Order the compost and mulch from the cubic yards shown.

Worked examples

Project	Result
20 × 15 ft of roof, silt, 6 to 7 in deep, within 30 ft of the downspout	Drainage area 300 sq ft → 75 sq ft rain garden, 327 gal ponded, 178 gal of runoff from 1 in
30 × 20 ft of roof, sand, 8 in deep, within 30 ft	Drainage area 600 sq ft → 48 sq ft rain garden (600 × 0.08), 239 gal ponded
30 × 20 ft of roof, clay, 3 to 5 in deep, within 30 ft	Drainage area 600 sq ft → 258 sq ft rain garden (600 × 0.43), 804 gal ponded
30 × 20 ft of roof, silt, more than 30 ft from the downspout	Drainage area 600 sq ft → 36 sq ft rain garden (600 × 0.06), 157 gal ponded
25 × 20 ft of paving, silt, 6 to 7 in deep, within 30 ft (an extension of the roof method)	Drainage area 500 sq ft → 125 sq ft rain garden (500 × 0.25), 281 gal of runoff from 1 in at 0.90

Frequently asked questions

How big should a rain garden be?: Multiply the drainage area by a size factor from the University of Wisconsin-Extension: 0.19 to 0.43 for a garden 3 to 5 in deep within 30 ft of the downspout, 0.08 to 0.20 for 8 in deep, depending on the soil. For 300 sq ft of roof on silty soil, 6 to 7 in deep, that is 75 sq ft; with sandy soil 8 in deep it would be 24 sq ft, and with clay 3 to 5 in deep 129 sq ft.
Why does soil change the size?: Water soaks into sand quickly and into clay slowly, so a clay garden has to be larger to hold the same runoff while it drains. Take a 600 sq ft roof at 6 to 7 in deep: 90 sq ft on sand, 150 sq ft on silt and 192 sq ft on clay. Soil that will not drain at all is a reason to pick another spot, not to build a bigger garden.
What if the garden is more than 30 feet from the downspout?: The Wisconsin tables use a different, much smaller factor: 0.03 for sandy soil, 0.06 for silty and 0.10 for clayey, for any depth from 3 to 8 in. A 600 sq ft drainage area on silt then needs 36 sq ft instead of 150 sq ft (6 to 7 in deep, within 30 ft). Pick the distance in the calculator. A long pipe or swale from the downspout is a way to get the water there.
How do I find the drainage area?: Measure the footprint of the roof section, or the paving, that sends water to the garden, as you would on a plan seen from above: length × width, including the overhang. Do not use the sloped area. If a gutter sends only part of the roof to one downspout, measure just that part. Take off any section that drains somewhere else with the “Add a part that drains elsewhere” button.
How many gallons does a rain garden hold?: Ponded volume = garden area × depth ÷ 12 × 7.48052 gallons per cubic foot. The 75 sq ft example ponds 327 gallons at 7 in. That is more than the 178 gallons that 1 in of rain sends off 300 sq ft of roof (300 × 1 ÷ 12 × 7.48052 × 0.95), because the garden also drains into the soil during a storm and the size factors cover bigger storms; the garden holds the runoff from about 1.8 in of rain before it overflows, ignoring the water that soaks in.
How much water runs off a roof in a 1 inch rain?: Area × 1 in ÷ 12 ft × 7.48052 gallons per cubic foot × the runoff coefficient. For a roof at 0.95, 1,000 sq ft gives 592 gallons; with nothing lost it is 623 gallons. A roof is close to a coefficient of 1 because little soaks in, and the calculator uses 0.95 as an estimate; use the 1.00 option if you want the worst case.
How do I test whether my soil drains well enough?: Dig a test hole where the garden will go (6 to 8 in deep is typical), fill it with water to soak the soil, then fill it again and see how long it takes to empty. Guidance varies: the Missouri Botanical Garden says it should empty within 24 hours (a rate of at least 0.25 in per hour), and the DC RiverSmart Homes worksheet allows up to 36 hours. Water standing longer than that means the spot does not drain well enough; try another spot. Run the test where you will dig, and at a time when the soil is not already wet or frozen. Your local extension office or stormwater program may set a different limit.
Can I use this for a driveway or a patio?: The Wisconsin manual is written for small roof areas, and says not to use it for parking lots, busy streets and other heavily used pavement without pretreatment. A driveway or patio can be entered here, using a coefficient of 0.90 for the runoff volume, but the size factor is the same as for a roof; treat the result as a rough starting point. For 500 sq ft of paving (25 × 20 ft) the garden is 125 sq ft and 1 in of rain gives 281 gallons. Oil, salt and silt in the runoff are why pavement needs more care.
How much compost and mulch does a rain garden need?: Garden area × the layer depth ÷ 12 ÷ 27 cubic yards. The 75 sq ft example needs 0.69 cu yd of compost mixed in at 3 in (25 bags of 0.75 cu ft) and 0.69 cu yd of mulch at 3 in (10 bags of 2 cu ft). The 2 to 4 in of compost and 2 to 3 in of mulch are typical figures, not rules; sandy soil may need little, and clay needs advice on grit and compost from your extension office. Use shredded hardwood mulch, which knits together and washes out less than chips.
What depth should I pick?: Deeper gardens are smaller: silty soil on 300 sq ft needs 102 sq ft at 3 to 5 in, 75 sq ft at 6 to 7 in and 48 sq ft at 8 in. The Wisconsin method covers depths from 3 to 8 in. Shallower is easier to dig and to keep safe for children and pets; deeper takes less room but holds more water for longer. Steep sites need more digging and a berm.