Rain Garden Size Calculator
Calculate the ideal rain garden dimensions based on your roof drainage area and soil type for effective stormwater management.
Rain Garden Dimensions
Rain garden sizing is based on your roof drainage area and how quickly your soil absorbs water.
Rain Garden Area = Drainage Area x Sizing Factor
Sizing factors by soil type:
- Sandy soil (drains fast): 20-30% of drainage area
- Loamy soil (moderate): 30-40% of drainage area
- Clay soil (drains slow): 40-60% of drainage area
Standard rain garden specs:
- Depth: 4-8 inches (10-20 cm) ponding depth
- Shape: Kidney or crescent shape works best
- Location: At least 10 ft from building foundation
- Slope: Garden should be level; berm on downhill side
Typical dimensions: A 1,000 sq ft roof with loamy soil needs about a 300-400 sq ft rain garden (roughly 15 x 22 ft).
Benefits:
- Reduces runoff by 30-40%
- Filters pollutants naturally
- Recharges groundwater
- Attracts pollinators with native plants