SCS Curve Number

Calculate runoff depth using the USDA SCS (NRCS) Curve Number method. Determine runoff, infiltration, and initial abstraction from precipitation and curve number.

Inputs

Precipitation

Curve Number (CN)

Tab to move between fields

Results

Runoff Depth

41.14 mm

Potential Max Retention (S)

84.67 mm

Initial Abstraction (Ia)16.93 mm

Infiltration41.93 mm

Runoff Ratio0.411

Retention Ratio0.419

How to Use This Calculator

Enter Precipitation and Curve Number (CN).
Review Runoff Depth (mm) and Potential Max Retention (S) (mm).
Use Initial Abstraction (Ia) (mm) and Infiltration (mm) to inform your decision.
Use the chart to visualize the results and explore different scenarios by adjusting inputs.

How the result changes with Curve Number (CN)

Curve Number (CN)	Runoff Depth	Potential Max Retention (S)
11	0 mm	2,055.09 mm
36	0.2 mm	451.56 mm
65	25.2 mm	136.77 mm
90	72.63 mm	28.22 mm

What each input means

Precipitation: Total storm rainfall depth in millimeters
Curve Number (CN): SCS curve number (30 = forest/good cover, 70 = residential, 98 = impervious pavement)

How this is calculated

Formula

S = 25400/CN - 254; Q = (P - 0.2S)² / (P + 0.8S)

Worked example, using the default values

Identify Input Parameters
Precipitation = 100, Curve Number (CN) = 75 = 2 input(s) provided
Calculate Runoff Depth
Runoff Depth
41.14 = 41.14
Calculate Potential Max Retention
Potential Max Retention = S
84.67 = 84.67
Calculate Initial Abstraction
Initial Abstraction
16.93 = 16.93
Calculate Infiltration
Infiltration = Math
41.93 = 41.93

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Hydrology

Rational Method Runoff

Calculate peak stormwater discharge using the Rational Method (Q = CIA/360). Estimate runoff from rainfall intensity, runoff coefficient, and catchment area.

Hydrology