Rain Garden Sizing Calculator

Quickly translate roof and patio runoff into a rain garden footprint that drains within a day, using soil absorption and ponding depth to right-size the basin.

Roof, driveway, and hardscape draining into the garden.
Rainfall depth for the target event or 24-hour storm.
Surface runoff factor; defaults to 0.90 if blank.
Optional. Defaults to 20 cm of surface storage.
Optional. Defaults to 25 mm/hr for loam.
Optional. Defaults to 24 hours to empty between storms.
Optional. Defaults to 0.85; use <1.0 when soil holds back water.

Examples

  • 120 m² roof, 35 mm design storm, runoff 0.95, 20 cm depth, 28 mm/hr infiltration, 24 h drawdown, storage 0.90 ⇒ Recommended rain garden 17.96 m²
  • 80 m² patio, 45 mm rain, runoff 0.75, 18 cm depth, 12 mm/hr infiltration, 36 h drawdown, storage 0.80 ⇒ Recommended rain garden 12.00 m²

FAQ

What if my soil drains slower than the defaults?

Lower the infiltration rate or extend the drawdown hours so the calculator increases the surface area accordingly.

Can I size the garden in multiple cells?

Yes. Use the recommended area as a total and divide it into separate basins connected by level spreaders or swales.

How do I account for mulch and planting layers?

Reduce the storage factor slightly (for example to 0.8) to reflect pore space taken up by mulch, root mass, and compost.

Additional Information

  • Catchment volume multiplies impervious area by storm depth and runoff coefficient, then adjusts for soil amendments with the storage factor.
  • Infiltration capacity limits ponding depth so the garden drains inside the allowed drawdown window.
  • Final area equals the required storage volume divided by usable depth, giving a plan-view footprint you can stake out on site.

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