Rain garden sizing calculator

Enter your roof or impervious surface area, your soil drainage class, and your local design rainstorm. Get the rain garden surface area, depth, and recommended infiltration rate. Method follows the EPA and Wisconsin DNR rain garden manual.

Roof or paved area draining to the garden (sq ft) Soil type Distance from downspout to garden Garden depth (typical 6–9 inches)

How rain garden sizing actually works

The method matches infiltration capacity to incoming runoff volume. From Wisconsin DNR and EPA Soak Up the Rain:

Step 1: Pick the design rainfall (typically 1 inch in 24 hr — the "first-flush" event that drives most pollutant loading).
Step 2: Multiply impervious area by sizing factor based on soil (a sandy loam factor of 0.25 means rain garden is 25% of the impervious area it serves).
Step 3: Subtract for distance from downspout (longer runs lose volume to absorption en route).
Step 4: Garden depth determines volume capacity; 6–9 inches is the operational range.

For sites with clay loam, the practical recommendation is to either amend the bottom 18 inches with sandy-loam compost mix (1:1 sand to compost with native soil) or move to a structural underdrain.

Plants to use

Rain garden plants need to handle both wet feet (4–48 hours after a storm) and drought between storms. Our full rain garden plant guide covers regionally appropriate species. Common bulletproof picks:

Bottom zone (wettest): Iris versicolor, Lobelia cardinalis (cardinal flower), Asclepias incarnata (swamp milkweed)
Mid zone: Rudbeckia fulgida (black-eyed Susan), Echinacea purpurea (coneflower), Panicum virgatum (switchgrass)
Edge zone: Schizachyrium scoparium (little bluestem), Solidago spp. (goldenrod), Aster novae-angliae (New England aster)