Rain Garden Size Calculator

A rain garden is a shallow, planted depression designed to capture and absorb runoff from impervious surfaces such as driveways, roofs, patios, and roads. Instead of rushing into storm drains (which carries pollutants into waterways), stormwater infiltrates through the rain garden soil and recharges groundwater.

Rain gardens are typically 20–30% of the area they are designed to drain. However, the exact size depends on three main factors: the size of the drainage area, the soil’s infiltration rate, and the depth of the garden.

The basic sizing formula:

Rain Garden Area = Drainage Area × Runoff Coefficient × Design Storm Depth / Soil Infiltration Rate / Ponding Time

A simplified practical approach (commonly used by extension services):

Rain Garden Area = Drainage Area × Runoff Factor / Depth Factor

Where the runoff factor accounts for how much of the rainfall from the drainage area actually runs off, and the depth factor accounts for how deep you make the garden (typically 15–30 cm / 6–12 inches).

Runoff factors, matching the options in the calculator:

• Roof: 0.95. Almost everything that lands on it leaves it.
• Driveway or paving: 0.90. Slightly less, because a little sits in the surface texture and evaporates.
• Mixed surfaces: 0.75. A typical suburban catchment of roof, path, and some planting.
• Compacted lawn: 0.60. Higher than people expect. An uncompacted lawn on good soil sheds only about 0.30, but the lawns that feed a rain garden are usually the ones that have been driven on, mown wet, or sit over builders’ rubble, and those shed far more. If yours genuinely soaks water away, you probably do not need a rain garden fed from it.

Soil infiltration rates:

• Sandy soil: very fast, 25–50 mm/hr (1–2 in/hr). Smallest garden needed.
• Sandy loam: fast, 12–25 mm/hr (0.5–1 in/hr). Good for rain gardens.
• Loam: moderate, 7–12 mm/hr (0.25–0.5 in/hr). Standard sizing.
• Clay loam: slow, 2.5–7 mm/hr (0.1–0.25 in/hr). Larger garden or amended soil needed.
• Clay: very slow, under 2.5 mm/hr. Rain gardens generally not suitable without significant soil amendment or underdrains.

A rain garden should drain completely within 24–48 hours after a rain event. If your native soil drains too slowly, amend with a mix of 50% sand, 30% topsoil, and 20% compost.

The garden should be placed at least 3 metres (10 feet) from the foundation of any building, and away from septic systems. It should be sited where it naturally collects water flow.

This calculator provides dimensions in both metric (square meters) and imperial (square feet).