Rain Garden Calculator

Calculate rain garden sizing for stormwater management. Determines required area, storage volumes, and infiltration capacity using ASCE and EPA methodology.

Calculate Rain Garden Size

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Contributing Drainage Area

Drainage Area

Total area draining to the rain garden

Impervious Percentage

Percent of drainage area that is impervious (0-100)

Pervious Ground Cover

Dominant ground cover type for pervious area

Design Storm

Design Rainfall Depth

Total rainfall depth for design storm (typically 1-2 year event)

Native Soil Properties

Soil Infiltration Class

Native soil type for infiltration rate

Rain Garden Design Parameters

Maximum Ponding Depth

Maximum water depth allowed in rain garden (typically 6-9 in)

Amended Soil Depth

Depth of engineered soil media (typically 18-24 in)

Amended Soil Porosity

Void fraction of amended soil media (typically 0.25-0.35)

Target Drawdown Time

hours

Maximum time to drain ponded water (24-48 hours for mosquito control)

Rain Garden Design Overview

Rain gardens (bioretention areas) capture and infiltrate stormwater runoff through ponding and amended soil media. They provide both water quality treatment and quantity control for the "first flush" of runoff.

Ponding Storage - Surface depression that temporarily holds water
Soil Storage - Water held in pore spaces of amended soil
Infiltration - Water percolates to native soil below
Drawdown Time - Time to drain (24-48 hours recommended)

Soil Infiltration Rates

Soil Class	Min (in/hr)	Typical (in/hr)	Max (in/hr)
Sand	2	8	12
Loamy Sand	1	4	8
Sandy Loam	0.5	2	4
Loam	0.25	1	2
Silt Loam	0.15	0.5	1
Sandy Clay Loam	0.1	0.3	0.5
Clay Loam	0.05	0.2	0.4
Silty Clay Loam	0.04	0.15	0.3
Silty Clay	0.02	0.1	0.2
Clay	0.01	0.05	0.1

Source: EPA SWMM Manual (2020), NRCS Soil Survey

Runoff Coefficients

Ground Cover	Min C	Typical C	Max C
Impervious Surface	0.90	0.95	0.98
Rooftop	0.85	0.90	0.95
Pavement/Concrete	0.85	0.90	0.95
Gravel Surface	0.25	0.50	0.70
Bare Soil	0.20	0.40	0.60
Lawn (Poor Condition)	0.25	0.35	0.45
Lawn (Good Condition)	0.10	0.20	0.30
Meadow/Grass	0.10	0.20	0.35
Forest/Wooded	0.05	0.15	0.25
Mixed Development	0.20	0.40	0.60

Source: USDA NRCS TR-55 (1986), ASCE MOP 77 (2006)