Rainfall Runoff Coefficient by Area Calculator

Runoff coefficient (C) by area

The runoff coefficient C is a dimensionless number: the share of rainfall that ends up as surface runoff. It anchors the Rational Method, Q = C · i · A, where Q is the peak discharge (m³/s), i the rainfall intensity (mm/h) and A the drainage area (ha or m²). What C really captures is the fraction of precipitation that runs off the surface rather than soaking in, evaporating, or getting caught up before it can flow.

Typical values run from C ≈ 0.05 on sandy soil under lawns up to C ≈ 0.95 on asphalt and steel roofs. When a catchment mixes several surfaces, you weight each one by its area: Ĉ = Σ(Cᵢ · Aᵢ) / ΣAᵢ. Most engineers reach for Brazilian standard NBR 10844 (rainwater drainage) or the SCS Curve Number method (USDA).

Applications

It shows up in urban stormwater design (gutters, culverts, detention basins), parking lot drainage, green roof sizing, and the hydrological impact studies that accompany development projects. Because C swings so much with land use, it becomes a pivotal number in low-impact development (LID) and SuDS planning.

FAQ

Why does C change with return period? Longer return periods mean more intense rain, and soil saturates sooner under it. To account for that, C is usually nudged up by a frequency factor Cᵢ (around 1.10 to 1.25 for TR ≥ 25 years).

Is the Rational Method valid for large basins? Not really. It assumes rainfall falls uniformly and is generally held to areas under 80 ha (200 acres). Once a basin is bigger than that, switch to SCS-CN, a unit hydrograph, or a distributed model like SWMM.

How does slope affect C? The steeper the ground, the less time water has to infiltrate, so C goes up. Tables in NBR 10844 and DAEE-SP break this out by slope band: flat < 2%, moderate 2 to 7%, steep > 7%.