Rational Method Calculator

Calculate peak discharge (Q) from rainfall data.

Calculated Intensity (I)

0.00 mm/h

Peak Discharge (Q)

0.00 m³/s

What is the Rational Method?

The Rational Method is a simple and widely used hydrologic technique for estimating peak discharge from small drainage basins. It relies on the principle that the maximum rate of runoff occurs when the entire watershed is contributing to the flow at the outlet.

Formula: Q = 0.278 × C × i × A; i = P × 60/t

C: Runoff coefficient (dimensionless, depends on land use and surface type)
I: Rainfall intensity (mm/h)
A: Drainage area (km²)

It is used to estimate peak runoff for drainage systems and hydraulic structures when its assumptions suit the catchment and the project requirements. There is no universal area limit: assess catchment size, response time, rainfall data and local design criteria before selecting the method.

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Q: m³/s · P: mm · t: minutes · i: mm/h · A: km². The coefficient includes unit conversion. FHWA — Highway Hydrology, §6.2

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