Rational Method Runoff

Calculate peak stormwater discharge using the Rational Method (Q = CIA/360). Estimate runoff from rainfall intensity, runoff coefficient, and catchment area.

Inputs

Rainfall Intensity

mm/hr

Runoff Coefficient (C)

Catchment Area

Tab to move between fields

Results

Peak Discharge

0.694 m³/s

Peak Discharge

694.44 L/s

Peak Discharge (Imperial)24.52 cfs

Flow Rate11,007.15 GPM

Runoff Volume (1hr Storm)2,500 m³

Effective Impervious %50%

How to Use This Calculator

Enter Rainfall Intensity, Runoff Coefficient (C), and Catchment Area.
Review Peak Discharge (m³/s) and Peak Discharge (L/s).
Use Peak Discharge (Imperial) (cfs) and Flow Rate (GPM) to inform your decision.
Use the chart to visualize the results and explore different scenarios by adjusting inputs.

How the result changes with Rainfall Intensity

Rainfall Intensity	Peak Discharge	Peak Discharge
51	0.708 m³/s	708.33 L/s
176	2.444 m³/s	2,444.44 L/s
325	4.514 m³/s	4,513.89 L/s
450	6.25 m³/s	6,250 L/s

What each input means

Rainfall Intensity: Design rainfall intensity for the chosen return period and time of concentration
Runoff Coefficient (C): Fraction of rainfall becoming runoff; 0.1 = forest, 0.5 = suburban, 0.9 = pavement
Catchment Area: Drainage area contributing to the design point in hectares (1 ha = 10,000 m²)

How this is calculated

Formula

Q = C × I × A / 360

Worked example, using the default values

Identify Input Parameters
Rainfall Intensity = 50, Runoff Coefficient (C) = 0.5, Catchment Area = 10 = 3 input(s) provided
Calculate Peak Discharge
Peak Discharge
0.694 = 0.694
Calculate Peak Discharge
Peak Discharge
694.44 = 694.44
Calculate Peak Discharge
Peak Discharge
24.52 = 24.52
Calculate Flow Rate
Flow Rate
11007.15 = 11007.15

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