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.

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.

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.

  • 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.

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