Ecology & Environment
Rain Garden Sizing Calculator
Estimate event runoff, infiltration depth, effective storage depth, base treatment area, safety-adjusted design area, footprint gap, site fraction, and static ponding volume.
Captured runoff volume (m³)-
Infiltration during drawdown (mm)-
Ponding plus drawdown infiltration depth (m)-
Base treatment area (m²)-
Area including safety factor (m²)-
Available footprint minus design requirement (m²)-
Design garden as contributing-area share-
Static ponding storage at design area (m³)-
Decision view
Rain-garden cross-section and footprint safety gauge
Rain-garden cross-section and footprint safety gaugeThe section separates ponding and drain-window infiltration while the target scale compares safety-adjusted area with available site space.
Exact scenario comparisonDesign infiltration rate (mm/h) changes while all other entered assumptions remain constant.
| Design infiltration rate (mm/h) | Captured runoff volume (m³) | Infiltration during drawdown (mm) | Ponding plus drawdown infiltration depth (m) | Base treatment area (m²) | Area including safety factor (m²) | Available footprint minus design requirement (m²) | Design garden as contributing-area share | Static ponding storage at design area (m³) |
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How to use Rain Garden Sizing Calculator
- Enter catchment area, rainfall, and runoff coefficient.
- Enter ponding depth, infiltration rate, and drain window.
- Enter the safety factor and available footprint.
- Review the cross-section and required-versus-available footprint gauge.
Calculator guide
Understanding Rain Garden Sizing Calculator
Rain-garden footprint depends on runoff volume and the water depth that can be managed through both surface ponding and infiltration during the drain window. This calculator makes that cross-section and the available-footprint decision visible.
Calculation method
How the calculation works
Translate captured runoff to volume and divide by the combined entered ponding and drawdown-infiltration depth, then apply an explicit area safety factor. Convert rainfall to catchment runoff volume, add ponding depth to drain-window infiltration depth, divide volume by that effective depth, and apply the safety factor.
Detailed calculation process
Convert catchment runoff into treatment depth and required footprint
The defaults route 30 mm rainfall from 450 m² at coefficient 0.85 into 150 mm ponding plus 12 mm/h infiltration over 24 hours.
General formula: V_r = A_c(P/1000)C; d_i=(i/1000)t; d_eff=d_p/1000+d_i; A_base=V_r/d_eff; A_design=F_s A_base; Gap=A_avail-A_design Rainfall depth over catchment area becomes runoff volume. Ponding and drain-window infiltration are expressed as metres of managed depth; volume divided by depth yields required plan area.
What each symbol means
Worked substitution with the default inputs
The default event produces 11.475 m³ runoff and a 32.7483 m² safety-adjusted rain-garden footprint, leaving 22.2517 m² within the available area.
Bioretention section
See water stored above and infiltrated below the surface
The engineering section separates ponding, media, infiltration arrows, and the site-fit decision.
Worked situations
Practical examples
- The default catchment produces 11.475 m³ runoff.
- Ponding plus drain-window infiltration provides 0.438 m modeled depth.
- The design footprint is 32.748288 m².
Better inputs
Useful tips
- Use a conservative field-tested infiltration rate.
- Apply local drawdown and pretreatment rules.
- Check overflow routing and setbacks even when the footprint fits.
Before relying on the result
Limitations and common mistakes
- The model assumes constant infiltration through the full drain window.
- Media storage, clogging, groundwater, side slopes, underdrains, inflow hydrograph, overflow, vegetation, and seasonal conditions are simplified.
- Site investigation and local stormwater criteria govern final design.
Reference
Key terms
- Ponding depth
- Temporary surface water depth above the garden media.
- Drain window
- Allowed time for modeled infiltration.
- Footprint gap
- Available area minus calculated design area.
Important note
Calculated from the entered environmental values using the displayed model. Measurement quality, local conditions, system boundaries, and source data affect interpretation.
Frequently asked questions
Why add ponding and infiltration depth?
The simplified volume balance treats both as water-management capacity over the drain window.
Does the model include media pore storage?
No. Only entered surface ponding and constant infiltration are included.
What does a negative footprint gap mean?
The calculated design area exceeds the entered available space.
Can infiltration rate be taken from a map?
Screening data can inform early planning, but final design should use appropriate site testing and safety factors.