Time of Concentration Calculator

Estimate the time required for runoff to reach the outlet point in a watershed using empirical hydrological methods.

Calculation Parameters

Length of the longest flow path
Average slope of the watershed (%)
Tip: For small rural catchments, use Kirpich method. For urban areas, use NRCS TR-55.

Calculate Time of Concentration

Enter your parameters and click "Calculate" to see results

Method Comparison

Compare results from different calculation methods using the same input parameters.

Method Time of Concentration Notes
Perform a calculation to see comparison results
Tip: Different methods may yield varying results. Choose the method most appropriate for your watershed characteristics.

Sensitivity Analysis

Tc vs Slope (Fixed Length)
Tc vs Length (Fixed Slope)

Reference Data

Manning's Roughness Coefficients (n)
Surface Type Manning's n Description
Asphalt 0.011-0.015 Smooth paved surfaces
Concrete 0.012-0.018 Finished concrete surfaces
Lawns 0.15-0.35 Short grass vegetation
Woods 0.30-0.80 Forested areas with underbrush
Natural Stream 0.03-0.05 Clean, straight stream
NRCS Runoff Curve Numbers (CN)
Land Use Hydrologic Soil Group CN
Urban (Residential) A 30-50
B 50-70
C 70-85
D 80-90
Woods A 25-45
B 55-70
C 70-80
D 78-83
Method Selection Guide
  • Kirpich Equation: Best for small rural watersheds (< 200 acres) with well-defined channels.
  • NRCS TR-55: Recommended for urban/suburban areas with mixed land use.
  • Manning's Kinematic Wave: Useful for overland flow analysis on different surfaces.

Last Updated: July 5, 2025

A few calculation issues fixed!