Rational Method Runoff Calculator (Peak Flow for Drains and Culverts)
Peak stormwater flow from a small catchment — a roof, car park, yard or field — from its runoff coefficient, rainfall intensity and area, in m³/s and L/s.
Results
The dashed line marks the value you entered.
What this tool does
The rational method, in use since the 19th century (Mulvaney 1851, Kuichling 1889), says the peak flow is Q = C × i × A, with the metric shortcut Q (m³/s) = C × i (mm/h) × A (ha) ÷ 360. The coefficient C is the fraction of rain that runs off: tables commonly give about 0.7–0.95 for roofs and paved areas and about 0.05–0.35 for lawns, depending on slope and soil. The rainfall intensity must be for a storm as long as the time the water takes to flow from the farthest point (the time of concentration) and for the return period you design for. The method is meant for small catchments, and the size limit varies by authority.
Formula
Q (m³/s) = C × i × A ÷ 360 (i in mm/h, A in hectares) · volume = Q × storm duration
Variables
| Symbol | Meaning | Unit |
|---|---|---|
cc | Runoff coefficient C (0 to 1) | — |
ii | Rainfall intensity i (for a storm as long as the time of concentration) | mm/h |
ha | Catchment area (1 ha = 10 000 m²) | ha |
du | Storm duration (use the time of concentration) | min |
Q | Peak runoff flow Q | m³/s |
QL | Peak runoff flow in litres per second | L/s |
VO | Runoff volume over the storm | m³ |
Worked example
- Runoff coefficient C (0 to 1)0.9
- Rainfall intensity i (for a storm as long as the time of concentration)60 mm/h
- Catchment area (1 ha = 10 000 m²)0.5 ha
- Storm duration (use the time of concentration)15 min
- Peak runoff flow Q0.0750 m³/s
- Peak runoff flow in litres per second75.0 L/s
- Runoff volume over the storm67.5 m³
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.