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Rational Method Calculator

Estimate stormwater peak runoff using the Rational Method.

Peak Runoff Discharge
Peak Runoff
—
m³/s
Result Value
Peak Runoff —
Peak Runoff —
Runoff Coefficient —
Rainfall Intensity —
Catchment Area —
Method: The Rational Method estimates peak runoff using Q = 0.278 × C × i × A, where Q is peak discharge in m³/s, C is the runoff coefficient, i is rainfall intensity in mm/hr, and A is catchment area in km². The rainfall intensity should correspond to the design storm duration, typically based on the catchment time of concentration.
Typical runoff coefficient ranges are provided for preliminary Rational Method calculations. The appropriate value should be selected based on actual site conditions and the applicable engineering design standard.
Area / Surface C Range
Roofs 0.75–0.95
Concrete / Impervious Pavement 0.70–0.95
Asphalt Pavement 0.70–0.95
Paved Areas 0.60–0.90
Gravel Areas 0.35–0.70
Lawns / Grass 0.10–0.35
Parks / Open Green Areas 0.10–0.30
Residential Areas 0.30–0.70
Commercial Areas 0.70–0.95
Industrial Areas 0.50–0.90
Agricultural / Open Land 0.10–0.40
Dense Urban Areas 0.70–0.95
Important: Runoff coefficients are not universal fixed values. The appropriate coefficient depends on surface characteristics, soil, slope, land use, imperviousness and the design guidance applicable to the project. The values shown here are typical preliminary ranges and should not replace project-specific engineering judgment or applicable standards.

Rational Method Formula

The Rational Method is commonly used to estimate peak stormwater runoff from a catchment. The calculation uses the runoff coefficient, rainfall intensity and catchment area.

In SI units, the Rational Method formula is: Q = 0.278 × C × i × A, where Q is peak discharge in m³/s, C is the runoff coefficient, i is rainfall intensity in mm/hr, and A is catchment area in km².

How to Calculate Peak Runoff

To estimate peak runoff, enter the appropriate runoff coefficient, design rainfall intensity and catchment area. The calculator converts the inputs to compatible units and calculates the resulting peak discharge.

The rainfall intensity used should correspond to the design storm duration, typically based on the catchment's time of concentration.

Rational Method Calculator FAQs

What is the Rational Method?

The Rational Method is a rainfall-runoff method used to estimate peak discharge from a relatively small catchment using rainfall intensity, runoff coefficient and drainage area.

What is the Rational Method formula?

In SI units, the formula is Q = 0.278 × C × i × A. Q is peak discharge in m³/s, C is the runoff coefficient, i is rainfall intensity in mm/hr, and A is catchment area in km².

What is the runoff coefficient?

The runoff coefficient represents the proportion of rainfall that contributes to surface runoff. Its value depends on factors such as surface type, land use, soil characteristics and catchment conditions.

What rainfall intensity should be used?

The design rainfall intensity should be selected for the appropriate return period and storm duration. For the Rational Method, the duration is commonly related to the catchment time of concentration.

What units does the calculator provide?

The calculator provides peak runoff in m³/s, litres per second (L/s) and cubic feet per second (cfs).