Last updated: 2026-06-21

Rain Garden Size

A rain garden size calculator works out how large a planted, sunken bed needs to be to soak up the stormwater running off a roof or paved area. Enter the area draining into it, the runoff coefficient of that surface, and your soil type, and it returns the recommended garden area. Rain gardens catch runoff and let it infiltrate slowly instead of rushing into drains, which eases flooding and filters pollutants — this tool sizes one for your site.

How a rain garden is sized

The garden has to handle the runoff from the surface feeding it. That runoff is the drainage area scaled by its runoff coefficient — the share of rain that runs off rather than soaking in. How large a garden that runoff needs depends on the soil: fast-draining sand empties quickly and needs less area, while clay holds water and needs more. A soil factor captures this:

where $A_g$ is the rain garden area, $A_d$ the drainage area, $c$ the runoff coefficient, and $k$ the soil factor — about 0.19 for sandy soil, 0.34 for loam, and 0.43 for clay at a typical ponding depth.

Worked example

A 50 m² roof, which sheds about 90 percent of its rain, draining onto loam soil:

So a rain garden of roughly 15 m² would handle the roof's runoff. On sandy soil the same roof would need only about 9 m², while on clay it would need closer to 19 m² — the soil, not just the roof size, drives the result.

Siting and limits

Put the garden in a natural low point or at the foot of a downspout, but keep it at least a few metres from foundations, and away from buried services and septic fields. The soil factors here assume a standard ponding depth; a deeper basin can be smaller in plan, and very slow clay soils may need an underdrain. Local rainfall and stormwater rules vary, so check your regional guidance and test how fast your soil actually drains before building. To capture some of that roof water for reuse instead, see Rainwater Harvesting Calculator.