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Rain Garden Sizing Calculator
Estimate runoff capture, ponding storage geometry, freeboard footprint, and a simplified infiltration drawdown time.
Concept screening only—not a permit, engineered design, or site approval. Confirm local stormwater rules and design storms; perform site-specific soil/infiltration testing; investigate groundwater, bedrock, utilities, contamination, foundations, wells, septic systems, property setbacks, slopes, and safe overflow routing. Actual infiltration can decline through clogging or saturation. Engage qualified local professionals where required.
Drainage surfaces
Area units: sqft, m2, acre. Coefficient must be 0–1 and should come from applicable local guidance. Up to 100 surfaces.
Applied to calculated runoff volume. Use only a fraction supported by the governing design method.
Equations used
Runoff volume = Σ(surface area × runoff coefficient) × rainfall depth × capture fractionRectangular frustum storage = depth ÷ 3 × (bottom area + top area + √(bottom area × top area))Top length/width = bottom length/width + 2 × side slope × vertical depthSimplified drawdown = ponding depth ÷ tested infiltration rateFreeboard is excluded from credited water storage. Infiltration during the storm is not subtracted from required storage. Drawdown is a screening ratio that ignores changing head, side infiltration, soil layering, clogging, groundwater, and outlet behavior.
Sizing result
—captured runoff
—bottom area
—water-line top area
—drawdown screen
| Surface | Area | Coefficient | Weighted area | Captured runoff |
|---|