Rainfall Runoff Coefficient by Area Calculator
Computes the weighted average rainfall runoff coefficient from the areas and partial coefficients of each surface type.
—
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%.
Related Tools
Bolt Tensile Stress Area (Metric)
Calculate the tensile stress area of a metric-thread bolt, A_t = (π/4)·(d − 0.9382·p)², from the nominal diameter d (mm) and the thread pitch p (mm). The tensile stress area is the EFFECTIVE cross-section resisting tension in a threaded bolt — and it is NOT the nominal-diameter area (the smooth cylinder) nor the root-diameter area (the thread bottom). Because of the helical thread geometry, tensile rupture occurs at an intermediate section, and tests showed it corresponds to an effective diameter equal to the average of the pitch and root diameters, leading to the formula with the 0.9382·p term (a geometric constant of the ISO metric thread, 60° triangular profile). The tensile area is the fundamental parameter for all bolt strength calculations: preload, tensile stress, proof load and ultimate strength are all found by multiplying A_t by the corresponding material stress. Using the wrong area (the larger nominal-diameter one) would overestimate strength and lead to undersized joints. Bolt tables list A_t for each diameter-pitch combination; this formula computes it for any metric thread. Enter the nominal diameter and the thread pitch.
Hexagon Area Calculator
Calculate the area of a regular hexagon from its side length. Instant result in the browser.
Body Surface Area (Mosteller)
Compute BSA by Mosteller formula: BSA = √(h·w/3600).
NR-23 Fire Extinguishers Area
Estimates fire extinguishers required by floor area per Brazilian NR-23 (1 per 500 m2).
VSWR (Standing Wave Ratio)
Calculate the voltage standing wave ratio (VSWR), VSWR = (1 + |Γ|) ÷ (1 − |Γ|), from the magnitude of the reflection coefficient |Γ|. The result (dimensionless, expressed as X:1) measures how well an antenna or load is matched to the transmission line: VSWR = 1:1 is a perfect match (all power delivered); higher values mean part of the wave is reflected back, forming standing waves that reduce efficiency and may damage the transmitter. Typically VSWR up to 1.5:1 or 2:1 is acceptable. Enter the magnitude of the reflection coefficient.
Tile Grout Quantity Calculator (kg)
Area in m2 plus square tile side, joint width and joint depth estimate how much grout the job takes, at a fixed density of 1.6 g per cm3.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.