Rational Method Calculator

A 25 ha catchment with a runoff coefficient of 0.6 under 50 mm/h of rain produces a peak runoff of 2.08333 m³/s.

Runoff coefficient C:

Roofs and pavement 0.7–0.95, lawns 0.05–0.35, cultivated land 0.2–0.6.

Rainfall intensity i (mm/h, for a duration equal to the time of concentration):

Catchment area A:

Peak runoff Q =

2.08333333333333 m³/s

Precision

Full answer, then cut to fewer decimals (not rounded):

2.08333333333333 m³/s
2.0833333333333 m³/s
2.083333333333 m³/s
2.08333333333 m³/s
2.0833333333 m³/s
2.083333333 m³/s
2.08333333 m³/s
2.0833333 m³/s
2.083333 m³/s
2.08333 m³/s
2.0833 m³/s
2.083 m³/s
2.08 m³/s
2.0 m³/s

In L/s: 2083.333333 L/s
In cusecs: 73.57222234 ft³/s

Step-by-step solution

Q = C i A = 0.6 × 50 mm/h × 25 ha
Q = C i A / 360 (A in ha) = 2.083333333 m³/s

Rational Method Formula

Q = C i A / 360

Q in m³/s, i in mm/h, A in ha (Q = CiA/3.6 with A in km²)

Worked Example

A 25 ha catchment with C = 0.6 and a design intensity of 50 mm/h:

Q = C i A = 0.6 × 50 mm/h × 25 ha
Q = C i A / 360 (A in ha) = 2.083333333 m³/s

In cusecs: 73.57222234 ft³/s

GATE tip

The rational method suits small catchments (up to about 50 km²); for larger basins use unit hydrographs.

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Frequently asked questions

Which rainfall duration should I use?

The duration equal to the time of concentration, when the whole catchment contributes at once.

How do I combine areas with different C?

Use the area-weighted coefficient C = ΣCᵢAᵢ/ΣAᵢ.

Sources

The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures .

Last reviewed: 2026-10-15. Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.

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Scientific Converters. (2026, October 15). Rational Method Calculator. Retrieved October 29, 2026, from https://scientificconverters.com/calculators/rational-method-calculator/

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Scientific Converters, "Rational Method Calculator," scientificconverters.com. Accessed: Oct. 29, 2026. [Online]. Available: https://scientificconverters.com/calculators/rational-method-calculator/