Sizing logic
Runoff gallons = drainage area x rainfall inches x 0.623 x runoff factor. Garden area = runoff cubic feet / ponding depth, plus the buffer.
RainYard Tools
Planted storage
Rain Garden Size Calculator
Estimate a rain garden surface area from drainage area, rainfall depth, runoff, ponding depth, and a sizing buffer.
Suggested area132 sq ftSurface area for the selected ponding depth.
Runoff volume449 galWater captured from the drainage area.
Storage needed60.0 cu ftEquivalent storage volume.
Example
An 800 sq ft roof area, 1 inch of rain, 90% runoff, 6 inches of ponding depth, and a 10% buffer gives about 132 sq ft of rain garden surface.
When the number is too large
Split the flow between two smaller gardens, reduce the drainage area, add a barrel first, or use an overflow path to a second practice.
What this calculator does not check
It does not verify soil infiltration, setbacks, utilities, groundwater, basement risk, or local permit rules.
Questions
Common checks.
Is a bigger rain garden always better?
No. It still needs the right soil, stable overflow, and plants that can handle wet and dry periods.
Can I use it for driveway runoff?
Yes for early sizing, but pavement water may carry sediment and needs a stable inlet.
What rainfall depth should I start with?
For a quick homeowner screen, 1 inch is a practical start. Use local requirements for regulated work.
Helpful referencesEPA green infrastructure typesEPA National Stormwater Calculator