How big should your rain garden be?

A rain garden does one job: it keeps roof runoff on your property and lets it soak into the ground. How big it must be depends on how much roof drains to it, how fast your soil takes in water and how deep you dig. This calculator uses the size factors from the University of Wisconsin-Extension manual, which Nebraska Extension repeats, and adds a storm and drain-down check.

What the number means

A rain garden is a shallow, planted basin that takes the runoff from a roof or lawn and lets it soak into the ground instead of the storm drain. It is not a pond, and it is not a rain barrel: the water is meant to be gone within a day or two. The rainwater harvesting calculator on this site figures how much water you can store for later use; this one finds how much ground you need to take the water away.

Method and variables

Drainage area = roof area × the share that flows to the downspout (many houses have four downspouts, each taking about a quarter of the roof), plus, for a garden more than 30 feet away, the lawn that drains to it. Garden area = drainage area × size factor. The factor comes from two tables of the Wisconsin manual: table 1 for a garden within 30 feet of the downspout (by soil and a depth of 3 to 5, 6 to 7 or 8 inches), and table 2 beyond 30 feet (by soil only). The storm check multiplies the drainage area by the rain depth and the share that runs off, then compares it with the garden's capacity, area × depth. The drain-down time is the depth divided by the infiltration rate.

Worked example: the manual's house

A 60 by 40 foot house has a 2,400 square foot roof. A downspout that takes a quarter of it drains 600 square feet. On silty soil with a 6 inch garden within 30 feet, the factor is 0.25, so the garden is 150 square feet. It holds 561 gallons when full. A 1 inch storm sends 374 gallons, so the garden has 1.50 times what is needed; at an infiltration rate of 0.5 inch per hour it empties in 12 hours. Per the manual, building it 70% of this size, 105 square feet, would still capture about 90 percent of average runoff.

Common mistakes and limits

The factors in the manual are for average rainfall in Wisconsin and Nebraska; a region with intense storms may need more storage, and the tables do not tell you. A garden placed where water already stands will not drain, however large it is. Digging deeper than 8 inches does not buy more capacity if the water then stays for days. Soil can be very different from the soil map for the plot: do the 6 inch hole test where you plan to dig. If the soil takes in less than 0.25 inch per hour, the Nebraska publication's threshold, a rain garden is the wrong tool without an underdrain or other design help. Check local rules on drainage onto neighboring land and on distance from the house, wells and septic systems.

How big should a rain garden be?

The garden's area is the area that drains to it times a size factor that depends on the soil and the depth. The University of Wisconsin-Extension manual (also used by Nebraska Extension) gives, for a garden within 30 feet of the downspout, 0.19 for sandy soil, 0.34 for silty and 0.43 for clayey soil at a depth of 3 to 5 inches, and smaller factors for deeper gardens. So the share is not a single "20 to 30 percent" of the roof: within 30 feet it runs from 8% (sandy soil, 8 inches deep) to 43% (clay, 3 to 5 inches). The manual's own example is a house of 60 by 40 feet with a downspout draining a quarter of the roof, 600 square feet, on silty soil with a 6 inch garden: 0.25 × 600 = 150 square feet.

Why does clay soil need a bigger garden?

Because water soaks into it more slowly, so the garden has to spread over more ground to take the same runoff. The manual says plainly that sandy soils infiltrate fastest and clayey soils slowest. For the example drainage area and a 6 inch garden the area is 90 square feet in sandy soil, 150 in silty soil and 192 in clayey soil. If the soil is too tight, the manual's test says to give up the site: a 6 inch hole filled with water that has not emptied after 24 hours is not suitable.

How deep should a rain garden be?

Between 4 and 8 inches, and never deeper than 8, because a deeper garden takes too long to drain (Nebraska Extension). The manual picks the depth from the slope of the lawn: under 4%, a 3 to 5 inch garden; 5 to 7%, 6 to 7 inches; 8 to 12%, about 8 inches. A deeper garden can be smaller: in the example, 4 inches needs 204 square feet, 6 inches 150 and 8 inches 96. The depth the calculator asks for is the ponding depth, the depth of water the garden holds before it overflows.

How long should water stay in a rain garden?

Nebraska Extension says standing water should drain within 24 to 48 hours, and asks for a soil that takes in at least 0.25 inch per hour. The time is the depth divided by the infiltration rate: a 6 inch pond in soil that takes 0.5 inch per hour is gone in 12 hours, but at 0.1 inch per hour it takes 60 hours, far too long. Measure the rate with a percolation or infiltration test in the spring or the rainy season rather than guessing; the calculator only does the division.

How much rain can the garden hold?

Its capacity is the area times the depth: 150 square feet at 6 inches is 561 gallons (one inch of water on one square foot is 0.6234 gallon). The roof area draining to the garden in the example sends 374 gallons for every inch of rain if all of it runs off. The sizing tables do not use a design storm: Nebraska Extension says the garden is designed to control 100 percent of the average rainfall. You can enter a storm depth and a runoff share to see how a bigger storm compares; both are your inputs, not a standard. Water above the ponding depth overflows, so plan where it goes.

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

Then the manual adds the lawn that drains to it to the roof area and uses a much smaller factor, 0.03 for sandy, 0.06 for silty and 0.10 for clayey soil, for any depth. If you pipe the roof water underground to the garden, size it as if it were next to the house. With 1,000 square feet of lawn added to the example, silty soil gives 96 square feet.

Where should a rain garden go?

At least 10 feet from the house so that infiltrating water does not seep into the foundation, not over a septic system, and not in a spot where water already stands: wet patches show where infiltration is slow, and a rain garden depends on it. Full or partial sun is better than under a big tree, and a flatter site is easier to dig. If the calculated area is more than about 300 square feet, the manual says to divide it into smaller gardens.

Sources: University of Wisconsin-Extension and Wisconsin Department of Natural Resources, “Rain Gardens: A How-To Manual for Homeowners” (GWQ037), tables 1 and 2 and the site and soil tests; University of Nebraska–Lincoln Extension, “Stormwater Management: Rain Garden Design for Homeowners” (G1758, extensionpubs.unl.edu), table I and the infiltration and drain-time guidance, all read October 27, 2026. Rutgers NJAES FS1175 (72 hours for mosquitoes) could not be opened and is not used. Check your local code for drainage rules. Storing the water instead: rainwater harvesting calculator; moving it away: French drain calculator; the soil to fill the garden: raised bed soil calculator; mulch: mulch calculator. How we check our calculators.