What is a runoff coefficient?

The runoff coefficient (C) is a dimensionless ratio between 0 and 1 that represents the fraction of rainfall that becomes surface runoff. A value of 1 means all rain runs off; 0 means all rain infiltrates.

What is the rational method?

The rational method estimates peak stormwater flow as Q = C × i × A, where Q is peak flow (ft³/s or m³/s), C is the runoff coefficient, i is rainfall intensity, and A is the drainage area.

Why does slope affect the runoff coefficient?

Steeper slopes increase flow velocity and reduce the time water has to infiltrate the soil, so a greater fraction of rainfall runs off. Flat terrain allows more ponding and infiltration.

Can I use these C values for any soil type?

The tabulated values represent typical conditions. Sandy or highly permeable soils will have lower C values; clay-heavy soils will have higher ones. Use these as starting estimates and consult local hydrology guidelines for final design.

What does a composite C mean?

When a catchment has multiple land cover types, you compute a weighted average C = Σ(Ci × Ai) / ΣAi, where Ci and Ai are the coefficient and area for each sub-area.

Are these values from a standard reference?

Yes. The ranges follow widely used engineering references including the ASCE Hydrology Handbook and standard civil engineering textbooks. The midpoint of each range is used as the representative value.

Runoff Coefficient Calculator

Name: Runoff Coefficient Calculator
Availability: InStock
Author: Nham Vu

Select land cover type and slope to get the rational method runoff coefficient (C) used in stormwater calculations.

Land Cover & Slope

Land Cover Type

Slope

Rational Method Formula

Q = C × i × A

QPeak runoff flow (ft³/s or m³/s)

CRunoff coefficient (dimensionless, 0–1)

iRainfall intensity (in/hr or mm/hr)

ADrainage area (acres or ha)

Runoff Coefficient (C)

Select land cover and slope, then press Calculate.

Composite C Calculator

Add multiple sub-areas to compute a weighted average C for mixed land cover.

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Summary

Select land cover type and slope to get the rational method runoff coefficient (C) used in stormwater calculations.

How it works

Select the land cover type that best describes your study area.
Choose the ground slope category (flat, moderate, or steep).
The runoff coefficient C is displayed instantly from standard engineering tables.
Use the displayed C value in the rational method formula Q = C × i × A.
Adjust inputs to compare C values for different land cover scenarios.

Use cases

Sizing storm drains and culverts for a new development site.
Estimating peak runoff flow for detention basin design.
Comparing runoff impact before and after land-use change.
Supporting hydrology reports for civil engineering permits.
Teaching rational method concepts in environmental science courses.
Quick field estimates during site reconnaissance.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu