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
Your order will appear here
Enter the project details and calculate a rounded material estimate.
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.
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.