Hydrologic Curve Number Calculator

Estimate runoff potential based on land use, soil type, and hydrologic conditions using the SCS/NRCS method

Input Parameters

Select the primary land use type for your area
Select more specific land use category
Select soil type based on infiltration characteristics
Select the condition of the land cover
0% 100%
Adjust the percentage of impervious surfaces (roads, roofs, etc.)
Enter the size of the area in acres
Curve Number Results
Calculated Curve Number (CN)

--

Potential Maximum Retention (S)

-- inches

Interpretation

Results will appear here after calculation.

Tip: Lower CN values indicate higher infiltration and lower runoff potential.
CN Range Runoff Potential
30-40 Very Low
40-55 Low
55-70 Moderate
70-80 High
80-100 Very High
Input Summary
  • Land Use Type --
  • Land Use Subtype --
  • Hydrologic Condition --
  • Soil Group --
  • Impervious Area --
  • Area Size --
Composite Curve Number Calculator

Calculate a weighted curve number for multiple land areas with different characteristics. This approach is similar to the method used in stormwater runoff estimation for complex sites.

User Guide & Methodology
About Curve Numbers

The Curve Number (CN) method is a widely used empirical model for estimating direct runoff from rainfall. Developed by the USDA Natural Resources Conservation Service (NRCS), it provides a simple approach to predict runoff based on land use, soil type, and hydrologic condition. For related applications, you might also explore the time of concentration calculator to better understand flow timing across your watershed.

How to Use This Tool
  1. Select the land use type and subtype from the dropdown menus
  2. Choose the hydrologic soil group (A, B, C, or D)
  3. Select the hydrologic condition (good, fair, or poor)
  4. Adjust the impervious area percentage if applicable
  5. Enter the area size in acres
  6. Click "Calculate Curve Number" to see results
Key Formulas
SCS Runoff Equation

Q = (P - 0.2S)² / (P + 0.8S)   for P > 0.2S

Where:

  • Q = Runoff depth (inches)
  • P = Rainfall depth (inches)
  • S = Potential maximum retention after runoff begins (inches)
Potential Maximum Retention (S)

S = (1000 / CN) - 10

Frequently Asked Questions

Refer to the "Soil Group Info" tab for detailed descriptions of each soil group. You can also consult soil surveys from the NRCS or local agricultural extension services. For foundation-related work, understanding soil bearing capacity is equally important for site planning.

  • Good: Dense vegetation cover, well-maintained, minimal compaction
  • Fair: Moderate vegetation cover, some compaction
  • Poor: Sparse vegetation, heavily compacted, poor infiltration

When you specify an impervious percentage, the tool calculates a weighted CN where the impervious areas are assigned CN=98 and the pervious areas use the selected CN value. The final CN is a weighted average based on the impervious percentage.
Hydrologic Soil Group Information

The four hydrologic soil groups are defined based on the soil's infiltration characteristics when thoroughly wet. These classifications also influence other geotechnical properties, such as those analyzed in the allowable bearing pressure calculator for shallow foundation design.

Group Description Infiltration Rate Examples
A High infiltration (low runoff potential) > 0.3 in/hr Deep sand, loamy sand
B Moderate infiltration 0.15 - 0.3 in/hr Sandy loam, silt loam
C Slow infiltration 0.05 - 0.15 in/hr Clay loam, sandy clay
D Very slow infiltration (high runoff potential) < 0.05 in/hr Clay, heavy clay
Note: Soil group determination should be based on the most restrictive layer within the upper 60 inches of soil.
Determining Your Soil Group

To accurately determine your soil's hydrologic group:

  1. Consult the USDA NRCS Web Soil Survey (websoilsurvey.nrcs.usda.gov)
  2. Check with your local agricultural extension office
  3. Review soil test results if available
  4. Perform infiltration tests if detailed information is needed
Practical Site Planning Guidance
When to Use This Tool
  • Pre-construction: Stormwater management planning and permitting
  • Site grading: Designing drainage systems and swales
  • Erosion control: Sizing sediment basins and silt fences
  • Land development: Calculating runoff for subdivision design
  • Post-construction: Verifying as-built conditions match design
Field Measurement Prep Checklist
Before you calculate:
  • Walk the site to identify all land use types
  • Take soil samples or verify soil maps on-site
  • Measure impervious areas with tape or GPS
  • Note vegetation condition and compaction areas
  • Account for temporary construction changes
Real-World Application Tips
Common Site Mistakes
  • Overlooking compacted areas: Construction traffic creates more "Poor" condition areas than natural sites
  • Ignoring temporary conditions: Stockpiles and staging areas affect runoff during construction
  • Underestimating impervious: Include all paved areas, even gravel roads and parking
  • Using wrong soil group: Subsurface layers often control infiltration, not topsoil
Material & Logistics Planning
  • Drainage structures: Higher CN = larger pipes and catch basins needed
  • Erosion control: Plan for more temporary measures if CN > 70
  • Site sequencing: Phase earthwork to manage runoff during wet seasons
  • Cost impact: Each 10-point CN increase can double drainage material needs. Use the building cost estimator to budget for these adjustments.
Weather & Field Adjustments

Seasonal considerations:

  • Use higher antecedent moisture (Condition III) for winter/spring work
  • Account for frozen ground reducing infiltration in cold climates
  • Adjust for vegetation growth cycles when planning long projects
  • Consider local 10-year storm data for critical drainage design. The stormwater runoff calculator can help with these advanced scenarios.
Cross-Check Your Results
  1. Verify soil group with on-site infiltration test if uncertain
  2. Compare calculated CN with nearby similar developed sites
  3. Use the multi-area calculator for complex sites
  4. Add 5-10% safety margin for drainage system design
  5. Document all assumptions for permit submissions
Contractor Q&A

CN tables assume stable conditions. Construction sites are dynamic—temporary stockpiles, exposed subgrade, and compaction from equipment can increase runoff by 20-40%. Always use the "Poor" condition setting during active earthwork and adjust impervious percentages for temporary conditions. For volume calculations during grading, try the earthwork volume calculator.

Single area: Use for preliminary planning or homogeneous sites. Composite: Essential for final design of mixed-use developments, commercial sites with landscaping, or any project with multiple soil types. The composite method prevents underestimating runoff from critical areas.

  • Soil compaction: From equipment traffic increases CN by 5-15 points
  • Vegetation removal: Changes "Good" to "Poor" condition immediately
  • Subsurface layers: Clay or hardpan at shallow depth controls infiltration
  • Surface smoothing: Grading can reduce natural depression storage
Tool Limitations & Best Practices

This calculator uses standard NRCS CN values. For final engineering design:

  • Verify with local jurisdiction requirements—some areas use modified CN values
  • Combine with on-site infiltration testing for critical projects
  • Consider subsurface drainage systems that aren't captured in CN method
  • Use rainfall frequency data specific to your project location
  • Remember CN method assumes uniform rainfall distribution

Professional judgment required: This tool supports planning but doesn't replace site-specific engineering analysis.