Calculate peak stormwater runoff from any catchment using the Rational Method. Enter your runoff coefficient C, rainfall intensity i, and catchment area A — get peak flow Q in litres per second and m³/s instantly.
<\!-- ── Live calculator ── -->Where:
| Symbol | Parameter | Units | Notes |
|---|---|---|---|
| Q | Peak stormwater flow | m³/s | Maximum instantaneous discharge |
| C | Runoff coefficient | dimensionless | Ratio of runoff to rainfall, 0–1 |
| i | Rainfall intensity | m/s | Convert mm/hr ÷ 3,600,000 to get m/s |
| A | Catchment area | m² | 1 ha = 10,000 m² |
Select the C value that best matches your catchment's dominant land cover. For mixed catchments, use an area-weighted average.
| Land Cover | C (typical) | C range | Notes |
|---|---|---|---|
| Dense impervious (roofs, carparks) | 0.90 | 0.85–0.95 | Commercial/industrial rooftops |
| Roads and paving | 0.80 | 0.75–0.90 | Asphalt, concrete surfaces |
| Dense residential (terraced housing) | 0.70 | 0.60–0.80 | High site coverage, paved drives |
| Suburban residential (semi-detached) | 0.55 | 0.45–0.65 | Mixed lawns and paving |
| Low-density residential | 0.40 | 0.30–0.50 | Large gardens, gravel drives |
| Parkland / lawns | 0.25 | 0.15–0.35 | Maintained grass, flat-to-moderate slope |
| Woodland / forest | 0.15 | 0.10–0.25 | Dense canopy with leaf litter |
| Agricultural / pasture | 0.30 | 0.20–0.40 | Depends on crop type and tillage |
Given: Mixed residential site — 0.8 ha catchment, C = 0.45 (area-weighted average of roofs 0.90, lawn 0.25), 10-year return period storm i = 48 mm/hr.
Step 1: Convert units
i = 48 mm/hr = 48 ÷ 3,600,000 = 1.333 × 10⁻⁵ m/s
A = 0.8 ha = 0.8 × 10,000 = 8,000 m²
Step 2: Apply Q = CiA
Q = 0.45 × 1.333 × 10⁻⁵ × 8,000 = 4.8 × 10⁻² m³/s = 48 L/s
Step 3: Pipe sizing
With Q = 48 L/s on a 1% slope, Manning's equation (n = 0.013) requires a 300 mm minimum diameter pipe. Use the pipe sizing calculator →
Stormwater Pipe Sizing (Manning's equation) — Size the pipe to carry your Q value.
Earthwork Cut/Fill Volume — Prismoidal cross-section volumes from station data.
Retaining Wall Stability (EN 1997) — Overturning, sliding, bearing utilisation checks.
Interactive Civil Demo — All three calcs on a pre-seeded 0.8 ha site.