Calculator Inputs
Example Data Table
| Garden scenario | Area | Storm | Assumptions | Peak flow (Rational) | Runoff volume (CN) |
|---|---|---|---|---|---|
| Lawn to a yard drain | 200 m² | 25 mm/hr for 30 min | C ≈ 0.28, CN 70, λ 0.20 | ~0.00039 m³/s | ~0.75 m³ |
| Mixed beds with clay soil | 350 m² | 35 mm/hr for 45 min | C ≈ 0.45, CN 80, λ 0.20 | ~0.00154 m³/s | ~3.60 m³ |
| Pavers draining to a swale | 120 m² | 30 mm/hr for 20 min | C ≈ 0.78, CN 92, λ 0.05 | ~0.00087 m³/s | ~1.65 m³ |
| Small roof downspout | 60 m² | 40 mm/hr for 15 min | C ≈ 0.95, CN 98, λ 0.20 | ~0.00060 m³/s | ~0.95 m³ |
Formula Used
Used to estimate peak runoff rate for pipes, channels, and drain inlets.
- Q = C × I × A
- Metric conversion: Q(m³/s) = 0.00278 × C × I(mm/hr) × A(ha)
- Imperial conversion: Q(cfs) = 1.008 × C × I(in/hr) × A(acres)
Used to estimate runoff depth from a storm, then convert to volume.
- Storage term: S = 25400/CN − 254 (mm) or S = 1000/CN − 10 (in)
- Initial abstraction: Ia = λ × S
- Runoff depth: Q = (P − Ia)² / (P + (1−λ)S) when P > Ia
How to Use This Calculator
- Select your unit system and preferred calculation mode.
- Enter the catchment area that drains to your inlet or swale.
- For peak flow, enter rainfall intensity and choose how C is set.
- For runoff volume, enter storm depth and a Curve Number value.
- Press Calculate and review peak flow, depth, and volume results.
- Download CSV or PDF to share with your site notes.
Runoff drivers in garden catchments
Runoff increases as surfaces harden and slopes steepen. A 200 m² area at 15 mm/hr can produce small flows, but at 60 mm/hr the peak can be roughly four times higher. Use the surface pick-list to estimate a starting runoff coefficient, then refine it using observed ponding, infiltration, and mulch coverage.
Peak flow planning with the Rational method
The Rational approach estimates peak discharge from intensity, area, and coefficient. For many residential drainage fixes, designers start with C values near 0.25–0.45 for lawns, 0.70–0.85 for pavers, and 0.90–0.98 for roofs. Select an intensity tied to a return period you can justify. Pair intensity with a duration that matches the time of concentration for the drained path.
Runoff volume with Curve Number storage
Volume matters when sizing soakaway pits, rain gardens, and storage tanks. Typical Curve Numbers can range from about 61–74 for good-condition turf on permeable soils, 75–86 for compacted yards, and 92–98 for near-impervious cover. The calculator applies an initial abstraction ratio (λ) to represent interception and surface storage. As storm depth rises, the model shifts from abstraction losses to direct runoff more quickly.
Sizing outlets, swales, and conveyance
Use the peak result to screen inlet capacity and swale conveyance. If your peak is 0.0015 m³/s, a shallow grassed swale with mild slope may cope, while steeper grades need erosion checks and lining. Add a freeboard allowance and confirm where overflow will go if a grate blocks. Convert peak flow to a target pipe size only after confirming allowable velocity and downstream restrictions.
Sensitivity checks and reporting
Test a conservative and an optimistic scenario before building. Increase intensity by 20% and raise C or CN one class to see the safety margin. If results swing sharply, prioritize field data: measure contributing area, confirm downspout connections, and note soil infiltration after a 25–30 mm rainfall. Export CSV or PDF so assumptions, units, and results stay consistent across site reviews. Record chosen surface and soil class.
FAQs
Which method should I use for garden drainage?
Use Rational for peak flow checks at inlets, swales, and pipes. Use Curve Number for storage volume sizing such as rain gardens or soakaways. If unsure, run Both and compare conservative results.
How do I choose rainfall intensity and duration?
Use a local intensity–duration–frequency source if available. Pick a duration near your runoff travel time to the outlet. For quick yard paths, 10–20 minutes is common; for longer drainage lines, test 30–60 minutes.
What Curve Number is reasonable for my yard?
Healthy turf on permeable soil often falls around 61–74. Compacted lawns and mixed surfaces commonly land near 75–86. Impervious patios, roofs, and dense paving are typically 92–98. Adjust upward if puddling persists.
What does the initial abstraction ratio (λ) change?
λ represents early losses like wetting, interception, and surface storage. Lower λ produces more runoff for the same storm depth; higher λ produces less. Use 0.20 as a standard starting point, then bracket 0.05–0.30 for sensitivity.
Can I mix metric and imperial inputs?
Keep all inputs in one unit system per run. The calculator converts results consistently, but mixing units can create unrealistic peaks and volumes. If you have mixed-source data, convert area, intensity, and depth before entry.
Are results suitable for final engineering design?
This tool is a planning estimator. Final design should confirm site grading, soil infiltration, downstream constraints, and safety overflow routing. For regulated work, consult local standards and a qualified professional to validate assumptions and sizing.