SCS/NRCS Runoff Curve Number (CN) Calculator

Calculate direct runoff depth, retention, and initial abstraction with the SCS/NRCS runoff Curve Number method, or area-weight multiple land-cover and soil-group subareas into a composite CN.

Calculation Mode

Runoff Calculation: Calculate runoff depth (Q) from rainfall depth (P) and curve number (CN) using the SCS equation.

Input Parameters

Rainfall & Curve Number

Enter storm rainfall and watershed characteristics

Rainfall Depth (P)

Total precipitation depth for the design storm

Curve Number (CN)

Runoff curve number (1-100)

Advanced Options

Adjust calculation parameters

Initial Abstraction Ratio

Ratio of initial abstraction to maximum retention (Ia = lambda * S)

Antecedent Moisture Condition

Prior moisture condition affects CN adjustment

Drainage Area (Optional)

Provide area to calculate runoff volume

Drainage Area

acres

Contributing watershed area (optional - for volume calculation)

SCS Curve Number Method Overview

The SCS (now NRCS) Curve Number method estimates direct runoff from storm rainfall. The basic equation is:

Q = (P - I_a)² / (P - I_a + S)

Where:

Q = Runoff depth (in)
P = Rainfall depth (in)
I_a = Initial abstraction = 0.2S (in)
S = Maximum retention = (1000/CN) - 10 (in)

Common Curve Number Values

Common SCS curve numbers by land use and hydrologic soil group
Land Use / Cover	Group A	Group B	Group C	Group D
Commercial (85% imp.)	89	92	94	95
Residential (1/4 acre)	61	75	83	87
Lawn (good condition)	39	61	74	80
Pasture (fair)	49	69	79	84
Woods (fair)	36	60	73	79
Paved (impervious)	98	98	98	98

Source: USDA TR-55 (1986), Table 2-2. Values shown for AMC II (average conditions).

Antecedent Moisture Conditions

AMC I - Dry

Dormant season with less than 0.5" of rainfall in prior 5 days. Lowest runoff potential.

CN_I = 4.2 CN_II / (10 - 0.058 CN_II)

AMC II - Average

Normal conditions. This is the standard condition used in TR-55 tables.

Standard CN values from tables

AMC III - Wet

Growing season with more than 2.1" of rainfall in prior 5 days. Highest runoff potential.

CN_III = 23 CN_II / (10 + 0.13 CN_II)

For educational purposes only. Not a substitute for professional engineering judgment.