Professional Stormwater Runoff Calculator
Calculate runoff using SCS-CN and SWMM methods with custom CN support and all equations shown.
Select Calculation Method
Rainfall Parameters
Soil Hydrologic Group
A
Sandy
B
Loamy
C
Clayey
D
Heavy Clay
Land Use / Cover
Currently Selected
Row Crops β Straight Row
CN (Soil B)
72
Watershed Area
Calculation Results
Runoff Depth (Q)
β
mm
Runoff Volume
β
mΒ³
Runoff Coefficient
β
Q/P
Curve Number
β
Potential Retention (S)
β
mm
Initial Abstraction
β
mm
Water Balance Breakdown
Runoff vs Rainfall Relationship
Step-by-Step Calculation
Click “Calculate Runoff” to see detailed steps
Scenario Comparison
Compare runoff across all soil groups
Subcatchment Parameters (per SWMM Manual)
Characteristic width for overland flow routing
Rainfall & Depression Storage
Hydraulic Parameters (Manning’s n)
Infiltration (per SWMM Manual)
For pervious areas only (constant rate simplification)
SWMM Simulation Results
Total Runoff Depth
β
mm
Runoff Volume
β
mΒ³
Peak Flow Rate
β
mΒ³/s
Runoff Coefficient
β
Time to Peak
β
minutes
Total Infiltration
β
mm
Water Balance Breakdown
Runoff Hydrograph
Step-by-Step Calculation (per SWMM User’s Manual)
Click “Run SWMM Simulation” to see detailed steps per EPA SWMM methodology
SCS-CN Result
Empirical method
SWMM Result
Per EPA SWMM User’s Manual
Method Comparison Analysis
Run both calculators to see comparison
Equations Used in Calculations
All formulas implemented in the calculator with full variable definitions and unit specifications.
SCS-CN Method Equations
1 Direct Runoff Equation
Q = Direct runoff depth (mm)
P = Rainfall depth (mm)
Ia = Initial abstraction (mm)
S = Potential maximum retention (mm)
2 Simplified Runoff Equation (Ia = 0.2S)
Where: Ia = 0.2S (standard NRCS assumption)
Substituting Ia = 0.2S into Equation 1 gives this simplified form.
3 Curve Number to Potential Retention
CN = Curve Number (0 to 100)
S = Potential maximum retention (mm)
English units: S = 1000/CN β 10 (inches)
4 Initial Abstraction
Represents surface storage, interception, and infiltration before runoff begins.
Note: Recent studies suggest Ia = 0.05S for urban areas, but 0.2S remains the NRCS standard.
5 Runoff Coefficient
C = Runoff coefficient (dimensionless, 0 to 1)
Represents the fraction of rainfall that becomes direct runoff.
6 Runoff Volume
V = Runoff volume (mΒ³)
Q = Runoff depth (mm)
A = Watershed area (hectares)
Note: 1 hectare Γ 1 mm = 10 mΒ³
SWMM Method Equations (per SWMM User’s Manual v5.1)
1 Continuity Equation (Nonlinear Reservoir Approach)
SWMM Manual Volume I, Chapter 1 β Section: Subcatchment Runoff
dS/dt = Rate of change of storage in subcatchment (mΒ³/s)
i = Net inflow rate = rainfall rate β infiltration rate (m/s)
A = Subcatchment area (mΒ²)
Q = Outflow rate (mΒ³/s)
This is the fundamental differential equation solved at each time step in SWMM.
2 Outflow Equation (Manning’s for Overland Flow)
SWMM Manual Volume I, Chapter 1 β Section: Nonlinear Reservoir Routing
Q = Outflow rate (mΒ³/s)
n = Manning’s roughness coefficient (impervious or pervious)
W = Characteristic overland flow width (m)
d = Flow depth (m), approximated as (S β Sd)
S = Subcatchment slope (m/m)
Combined with continuity: Q = (1/n) Γ W Γ d^(5/3) Γ S^(1/2) for wide rectangular channel assumption.
3 Overland Flow Depth (from Storage)
SWMM Manual Volume I, Chapter 1
d = Average flow depth (m)
S = Total storage depth (m)
Sd = Depression storage depth (m)
A = Subcatchment area (mΒ²)
Flow only occurs when total storage S exceeds depression storage Sd.
4 Runoff from Impervious Areas
SWMM Manual Volume I, Chapter 1 β Section: Pervious/Impervious Division
Qimp = Impervious runoff rate per unit width (mΒ²/s/m)
dimp = Flow depth on impervious area (m)
nimp = Manning’s n for impervious areas
No infiltration occurs on impervious surfaces per SWMM methodology.
5 Runoff from Pervious Areas
SWMM Manual Volume I, Chapter 1
Qperv = Pervious runoff rate per unit width (mΒ²/s/m)
dperv = Flow depth on pervious area (m)
f = Infiltration rate (m/s)
nperv = Manning’s n for pervious areas
Infiltration rate f depends on the selected infiltration method (Constant, Horton, Green-Ampt, or SCS-CN).
6 Horton Infiltration Equation
SWMM Manual Volume I, Chapter 1 β Section: Horton Infiltration
f(t) = Infiltration capacity at time t (mm/hr)
fβ = Initial (maximum) infiltration capacity (mm/hr)
fc = Final (minimum/steady-state) infiltration capacity (mm/hr)
k = Decay coefficient (1/hr)
t = Time since start of rainfall (hr)
Modified Horton with recovery between storms is also available in SWMM.
7 Green-Ampt Infiltration Equation
SWMM Manual Volume I, Chapter 1 β Section: Green-Ampt Infiltration
f = Infiltration rate (mm/hr)
Ks = Saturated hydraulic conductivity (mm/hr)
Ο = Wetting front suction head (mm)
ΞΞΈ = Change in moisture content = porosity β initial moisture
F = Cumulative infiltration depth (mm)
Green-Ampt is the preferred infiltration method in SWMM for physically-based modeling.
8 SCS Curve Number Infiltration
SWMM Manual Volume I, Chapter 1 β Section: SCS Curve Number Infiltration
F(t) = Cumulative infiltration at time t (mm)
P(t) = Cumulative rainfall at time t (mm)
S = Potential maximum retention (mm)
SWMM applies SCS-CN to pervious sub-areas only, with Ia = 0.2S by default.
β Water Balance Equation
The fundamental hydrologic equation: All rainfall is partitioned into runoff, infiltration, interception, and surface storage.
P
Rainfall
Input
Q
Runoff
Surface flow
F
Infiltration
Into soil
Ia
Interception
Vegetation
S
Storage
Puddles, depressions
References & Sources
SCS-CN Method References
- [1]USDA Soil Conservation Service (1986). “Urban Hydrology for Small Watersheds”, Technical Release TR-55, 2nd Edition. Washington, D.C.
- [2]USDA NRCS (2004). “National Engineering Handbook, Section 4: Hydrology” (NEH-4), Chapter 10: Estimation of Direct Runoff from Storm Rainfall.
- [3]USDA NRCS (2007). “National Engineering Handbook, Part 630: Hydrology”, Chapter 9: Estimating Runoff Volume and Peak Discharge.
- [4]Ponce, V.M. and Hawkins, R.H. (1996). “Runoff Curve Number: Has It Reached Maturity?” Journal of Hydrologic Engineering, ASCE, Vol. 1, No. 1, pp. 11-19.
SWMM Method References
- [5]U.S. EPA (2015). “Storm Water Management Model Reference Manual Volume I β Hydrology”, EPA/600/R-15/162A. Chapter 1: Runoff Processes.
- [6]U.S. EPA (2015). “Storm Water Management Model Reference Manual Volume II β Hydraulics”, EPA/600/R-15/162B.
- [7]Rossman, L.A. (2015). “Storm Water Management Model User’s Manual Version 5.1”, EPA/600/R-15/162. National Risk Management Research Laboratory, Cincinnati, OH.
- [8]Huber, W.C. and Dickinson, R.E. (1988). “Storm Water Management Model, Version 4: User’s Manual”, EPA/600/3-88/001a. U.S. EPA.
General Hydrology & Engineering References
- [9]Chow, V.T., Maidment, D.R., and Mays, L.W. (1988). “Applied Hydrology”. McGraw-Hill.
- [10]McCuen, R.H. (2016). “Hydrologic Analysis and Design”, 4th Edition. Pearson.
- [11]Mays, L.W. (2010). “Water Resources Engineering”, 2nd Edition. Wiley.
- [12]Viessman, W. and Lewis, G.L. (2003). “Introduction to Hydrology”, 5th Edition. Pearson.
Manning’s Equation & Time of Concentration
- [13]Manning, R. (1891). “On the Flow of Water in Open Channels and Pipes.” Transactions of the Institution of Civil Engineers of Ireland, Vol. 20, pp. 161-207.
- [14]Kirpich, Z.P. (1940). “Time of Concentration of Small Agricultural Watersheds.” Civil Engineering, Vol. 10, No. 6, p. 362.
Note: All equations and CN values implemented in this calculator are based on the above references. Always verify results with local regulations and engineering standards.