DrainageCalculators

SCS Curve Number Calculator

Calculate stormwater runoff depth using the SCS (NRCS) Curve Number method. Includes runoff calculation, composite CN for multiple areas, and CN lookup tables. Free online calculator for drainage engineers.

What This Solves

Estimates total runoff depth from a rainfall event based on soil type, land cover, and antecedent moisture using the NRCS Curve Number method.

Best Used When

  • You need to estimate runoff volume for pond or detention basin sizing
  • You have mixed land uses and soil types across the drainage area
  • You are working with hydrologic soil group (HSG) data and NRCS land cover classifications

Do NOT Use When

Key Assumptions

  • The rainfall event is a single storm of uniform intensity
  • Initial abstraction (Ia) is a fixed ratio of maximum retention (typically 0.2S)
  • Soil moisture conditions are uniform across the watershed
  • The curve number adequately represents the combined effects of soil type, land use, and surface condition
  • Antecedent moisture condition remains constant during the storm

Input Quality Notes

Curve numbers are empirical values that vary with antecedent moisture, season, and land management. Verify CN selections against local calibration data or NRCS tables for your region.

Try a Common Scenario

Click to pre-fill the calculator with realistic values.

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

in

Total precipitation depth for the design storm

Runoff curve number (1-100)

Advanced Options

Adjust calculation parameters

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

Prior moisture condition affects CN adjustment

Drainage Area (Optional)

Provide area to calculate runoff volume

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 - Ia)2 / (P - Ia + S)

Where:

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

Common Curve Number Values

Land Use / CoverGroup AGroup BGroup CGroup D
Commercial (85% imp.)89929495
Residential (1/4 acre)61758387
Lawn (good condition)39617480
Pasture (fair)49697984
Woods (fair)36607379
Paved (impervious)98989898

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.

CNI = 4.2 CNII / (10 - 0.058 CNII)

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.

CNIII = 23 CNII / (10 + 0.13 CNII)

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

This disclaimer applies to the SCS Curve Number Calculator calculator and all associated calculations.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.