Rain Garden Size Calculator
Estimate preliminary rain-garden surface area, captured runoff volume, square-equivalent dimensions and excavation volume using editable rainfall, runoff and storage assumptions.
Default project preview
144 ft²
Captured runoff
539 gal
Square-equivalent side
12 ft
Excavation volume
8 yd³
Effective storage depth
6 in
Calculator
Build the material order
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Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Contributing drainage area
Enter this project assumption in ft². The current imperial default is 1200 ft².
Design rainfall event
Enter this project assumption in in. The current imperial default is 1 in.
Runoff coefficient
Enter this project assumption in factor. The current imperial default is 0.9 factor. Minimum: 0.05. Maximum: 1.
Event volume to capture
Enter this project assumption in %. The current imperial default is 80 %. Minimum: 1. Maximum: 100.
Effective storage depth
Enter this project assumption in in. The current imperial default is 6 in.
Amended soil depth
Enter this project assumption in in. The current imperial default is 18 in.
Excavation price
Enter this project assumption in per yd³. The current imperial default is 45 per yd³. Minimum: 0.
How to read the outputs
Captured runoff
coefficient applied. Default preview: 539 gal.
Square-equivalent side
planning geometry. Default preview: 12 ft.
Excavation volume
at amended soil depth. Default preview: 8 yd³.
Effective storage depth
entered assumption. Default preview: 6 in.
Runoff coefficient
fraction reaching garden. Default preview: 0.9.
Excavation allowance
entered unit rate. Default preview: $360.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Captured runoff = drainage area × rainfall depth × runoff coefficient × capture percentage. Garden area = captured volume ÷ effective storage depth.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: August 19, 2026
Convert a design rainfall event over a contributing drainage area into captured runoff volume, preliminary rain-garden surface area and excavation volume.
Calculation steps
- 1Measure the impervious/pervious area actually routed to the rain garden.
- 2Convert rainfall depth to runoff volume using an appropriate runoff coefficient.
- 3Apply the selected capture percentage.
- 4Divide captured volume by effective storage depth to estimate surface area, then use soil depth for excavation planning.
Worked numerical example
A 1,200 ft² drainage area receiving 1 in of rain at a 0.90 runoff coefficient produces about 90 ft³ of runoff. Capturing 80% gives 72 ft³; with 6 in effective surface storage, the preliminary garden area is about 144 ft².
Purchasing and rounding
Planting soil, mulch, underdrain, overflow and plants are separate from the simple excavation volume. Confirm local rain-garden design guidance and soil infiltration before ordering materials.
Interpret the result
The calculated footprint is a storage-based preliminary size. Soil infiltration, drawdown, setbacks, overflow routing and side slopes can require a different final geometry.
Page-specific assumptions
- Drainage area and runoff coefficient are representative.
- Storage depth is usable across the basin.
- One design rainfall event is evaluated.
Common mistakes
- Using total lot area instead of connected drainage area.
- Ignoring infiltration testing.
- Treating ponding depth as excavation depth.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
FAQ
Rain Garden Size Calculator questions
What is effective storage depth?+
It is the design volume stored per unit of garden surface, combining only the storage mechanisms appropriate to the planned section.
Does every soil drain fast enough?+
No. Infiltration testing and site conditions determine whether underdrains, overflow or another strategy is needed.
Can a rain garden receive roof runoff?+
Often yes, when conveyance, overflow and setbacks are properly planned and roof materials are suitable.