Garden & Water Utilities
Rain Garden Size Calculator
Estimate how much surface area a rain garden needs to hold the runoff from a roof or paved area. Every number is an input you control: storm depth, runoff coefficient, ponding depth, media and infiltration.
How the size is worked out
Runoff volume = catchment area × design storm depth × runoff coefficient. Garden surface area = runoff volume ÷ effective storage depth, where effective depth = ponding depth + media void fraction × media depth. Excavation = surface area × (media depth + mulch depth). Drain-down time = ponding depth ÷ infiltration rate. The estimate holds the whole storm in the garden and ignores infiltration during the storm, so it is on the cautious side.
The optional rule-of-thumb line multiplies the catchment area by the percentage you enter from your local extension sheet. For reference, Oregon State University Extension's fact sheet reports modelled footprints of 5.4% to 12.5% of the drainage area for six Oregon sites, sized for infiltration at 2 inches/hour under the 25-year, 24-hour storm; those percentages are not defaults here and do not transfer to other climates or soils.
Oregon State University Extension states that rain gardens are designed to drain within 30 hours for a Type IA storm distribution and 72 hours for Type I and Type II distributions, and that ponding depth may be up to 12 inches. The drain-down limit is editable; use the figure your local authority requires. Design storm depth and any required setbacks, overflow and permits are set locally, so check with your planning department. This is a planning estimate, not an engineered design.
Sources: EPA, Soak Up the Rain: Rain Gardens; OSU Extension, Rain Gardens fact sheet (from the rain garden downloads page); for long-term site modelling see the EPA National Stormwater Calculator.