Rain Garden Sizing Calculator

Introduction to rain garden sizing and the storage it depends on

A rain garden is not really a planting bed. It is a small, shallow detention basin that happens to be planted. Runoff arrives from a roof or a driveway in a few concentrated minutes, the basin holds it, and the soil underneath releases it over the following hours. Every sizing decision follows from that: the bed must be large enough to hold the runoff volume from your chosen design storm, and it must empty fast enough that the next storm finds it available.

What trips people up is where the storage lives. Most homeowner guidance counts only the ponded water above the soil surface, which is why so many rain gardens come out looking enormous. In a bed built with engineered bioretention media, a large share of the storage is in the pore space of the mix itself: an 18-inch layer at 30 % porosity holds another 5.4 inches of water depth that never shows on the surface. This calculator counts both, so a bed with a proper media profile comes out roughly half the footprint of one sized on ponding alone.

The page also does two things a plain area formula cannot. It converts your soil infiltration rate into a surface-pool drawdown time and flags it against the 24-to-48-hour window that most stormwater manuals require, and it cross-checks the volumetric answer against the University of Wisconsin–Extension size-factor tables, which are calibrated to a whole average rainfall year rather than a single storm. When those two numbers disagree by a factor of three, that disagreement is the useful information.

How to use the rain garden sizing calculator

Impervious area is the hard-surface footprint that actually drains to the bed, measured in plan view. For a roof, that means the building footprint served by the downspout you are routing, not the sloped roof surface. Most houses have four downspouts each taking roughly a quarter of the roof, so a 2,400 sq ft roof usually contributes about 600 sq ft per downspout.

Design rainfall depth is the storm you want captured. One inch is the common water-quality storm in much of the United States because it captures the majority of annual runoff volume, but if a local program specifies a different depth — 1.2 in, 1.5 in, or a 90th-percentile event from your rainfall records — enter that instead.

Runoff coefficient is the fraction of that rain that leaves the surface. Use 0.95 for shingle, metal or membrane roofs and for concrete and asphalt; 0.40–0.70 for gravel and open-jointed pavers; 0.15–0.35 for lawn depending on soil.

Ponding depth is the temporary pool above the mulch line, measured to the invert of the overflow. Four to eight inches is the residential norm.

Soil media depth and media porosity describe the engineered mix, if you are building one. A typical bioretention profile is 18–30 in of a sand-dominated compost blend; 0.30 is a conservative porosity and several state manuals allow 0.40. Leave the depth at 0 if you are simply excavating a saucer in native topsoil, because native soil at field capacity contributes almost no usable storage.

Soil infiltration rate is optional but worth entering. Measure it with a bucket in a test hole or take the design rate your soil report gives. The calculator divides ponding depth by this rate to report how long the pool takes to disappear.

Soil texture and distance from the downspout drive only the UW-Extension cross-check; they do not affect the volumetric result.

Press Calculate Area and you get the runoff volume in cubic feet and gallons, the equivalent storage depth your profile provides, the required footprint, that footprint as a percentage of the drainage area, the drawdown verdict, and a scaled cross-section drawing of the bed you have just specified.

Sizing formula: runoff volume, effective depth, and drawdown time

Step one is the runoff volume produced by the design storm. With the impervious area in square feet, the runoff coefficient and the rainfall depth in inches:

Formula: V = A ⁢ C ⁢ D / 12

The division by twelve converts inches to feet so the answer lands in cubic feet. To read it in gallons, multiply by 7.48052, the number of US gallons in a cubic foot:

Formula: V_gal = 7.48052 ⁢ V

Step two is the effective storage depth of the profile you intend to build. Ponding depth counts in full; media depth counts only through its porosity , because the solid fraction of the mix cannot hold water:

Formula: d_eff = (P + M ⁢ n) / 12

Step three divides one by the other. The required surface area is simply the volume the bed must hold divided by the depth it can hold it at:

Formula: A_garden = V / d_eff = (A ⁢ C ⁢ D) / (P + M ⁢ n)

Notice that the two twelfths cancel in the collapsed form, so the ratio of rainfall depth to storage depth is what matters, not the units they are expressed in — as long as both are in the same units. The footprint as a share of the drainage area follows directly:

Formula: R = A_garden / A = (C ⁢ D) / (P + M ⁢ n)

This ratio is independent of how big the roof is, which is why rules of thumb expressed as a percentage of drainage area work at all. Finally, the surface pool empties at the soil’s infiltration rate in inches per hour, giving a drawdown time:

Formula: t_draw = P / f

The target is  h in most manuals, with 48 h as an outer limit. This expression deliberately ignores the falling-head effect and any lateral seepage, both of which make the real basin empty slightly faster, so it errs on the safe side.

Worked example: a 1,000 sq ft roof and a one-inch design storm

Take 1,000 sq ft of shingle roof draining to one bed, a 1-inch design storm, a runoff coefficient of 0.95, 6 in of ponding, 18 in of bioretention media at 0.30 porosity, and a measured infiltration rate of 0.5 in/h.

The runoff volume is 1,000 × 0.95 × (1 ÷ 12) = 79.2 cubic feet, which is 79.2 × 7.48052 = 592 gallons. The effective storage depth is 6 + 18 × 0.30 = 11.4 inches, or 0.95 feet. Dividing gives 79.2 ÷ 0.95 = 83.3 square feet — a bed roughly 13 ft by 6.5 ft, or 8.3 % of the contributing roof area.

Now strip out the media. With ponding alone the effective depth drops to 0.5 ft and the required area doubles to 158.3 square feet, or 15.8 % of the roof. That single input is the difference between a bed you can fit beside a downspout and one that eats a third of a small back garden, which is why it is worth deciding early whether you are importing an engineered mix or just digging a saucer.

Drawdown: 6 inches of ponding at 0.5 in/h empties in 12 hours, comfortably inside the 24-hour target. Halve the infiltration rate to 0.25 in/h and the pool takes 24 hours — right at the limit, and a signal to either reduce ponding depth, amend the subgrade, or add an underdrain.

Cross-checking against the UW-Extension size-factor tables

The University of Wisconsin–Extension homeowner manual, the most widely copied residential rain garden reference in the United States, skips the volumetric arithmetic entirely. It multiplies the drainage area by a size factor chosen from soil texture, garden depth and distance from the downspout. Those factors are reproduced below and are what the cross-check row in the results uses.

For the worked example — silty soil, 6 in deep, within 30 ft of the downspout — the factor is 0.25, giving 250 square feet against the volumetric answer of 83.3. The gap is not an error in either method. The size factors are calibrated to capture essentially all runoff over an average Wisconsin rainfall year, not one 1-inch storm, and they assume a plain excavated bed in native soil with no engineered media doing any storage work. The manual itself notes that a garden built 30 % smaller than the tables suggest still controls close to 90 % of annual runoff, and that any bed working out much beyond 300 square feet is better split into several smaller ones.

Read the two numbers as a bracket. The volumetric result is the minimum that satisfies your stated design storm; the size factor is a generous annual-capture target. A bed somewhere between them, with a safe overflow route, is usually the practical answer.

Interpreting the result on a real site

The recommended area is a target, not a shape. A 10 × 16 ft rectangle, a kidney-shaped bed hugging a path, and a long shallow swale of the same square footage all store the same water, provided the bottom is genuinely level. An unlevel bottom is the single most common failure: water pools at the low end and spills over the lip long before the design volume is reached, so the effective storage is whatever depth the shallowest point provides.

If the drawdown check fails, the fix is rarely a bigger garden. Options in rough order of cost are: reduce ponding depth and widen the footprint, replace the subgrade with a sand-dominated mix, or install an underdrain that turns the bed into a filter rather than an infiltration basin. Compacted construction subgrade is a frequent culprit and can often be recovered by deep tilling before the media goes in.

Limitations of this rain garden sizing model

Static volume, not a routing model. The calculator assumes the whole design storm arrives before any of it infiltrates. Real storms deliver rain over an hour or more while the bed is already draining, so the true peak storage is somewhat less than the volume computed here. The assumption is conservative and standard for homeowner-scale sizing, but it is not a hydrograph.

Porosity is treated as fully available. Media pore space that is already occupied by antecedent moisture cannot store new runoff. Manuals that allow 0.40 generally assume a well-drained mix and a storm arriving on a reasonably dry bed. Entering 0.30 rather than 0.40 is a cheap way to buy margin.

Drawdown is a constant-rate estimate. Dividing ponding depth by infiltration rate ignores the declining head as the pool empties and any lateral flow, and it says nothing about how long the media itself takes to drain. It answers only the question the manuals ask, which is how long standing water persists.

No bypass, clogging or long-term siltation. Storms larger than your design event will exceed the bed, which is why a safe overflow route matters more than any number on this page. Surface clogging from fine sediment reduces infiltration over years and is the normal reason an established bed starts holding water.

Siting rules are yours to apply. Keep the bed at least 10 ft from the house so infiltrating water does not reach the foundation, stay clear of septic fields, retaining walls and utility runs, and avoid lawns steeper than about 12 % slope without professional advice. Do not site the garden in an existing wet patch: those mark where infiltration is already poor.

Sources. Size factors, depth guidance, the 24-hour test-hole screen and the 30 %-smaller/90 %-capture note: University of Wisconsin–Extension and Wisconsin DNR, Rain Gardens: A How-To Manual for Homeowners (publication GWQ037), Tables 1 and 2. Bioretention media porosity of 0.30–0.40 and the 24–48 hour surface-pool drawdown requirement: Minnesota Stormwater Manual, design criteria for bioretention. General practice context: US EPA, Soak Up the Rain: Rain Gardens. Unit conversion: 1 cubic foot = 7.48052 US gallons.

Planting zones and first-year maintenance

Plant choice follows the moisture gradient the basin creates. The bottom is saturated for hours after every storm and bone dry a week later, which is a punishing combination that native wetland-edge species handle far better than nursery ornamentals. The rim never floods and can take ordinary border plants.

During the first year the bed needs real attention: water new plants through dry spells, keep weeds from colonising the bare mulch, and after each heavy storm check the inflow point for erosion and confirm the pool has gone within a day. Once roots are established the maintenance settles into seasonal cutback, occasional weeding, and keeping the inflow and overflow paths clear.

Common questions about rain garden sizing