DrainageCalculators

Time of Concentration Calculator

Calculate time of concentration (Tc) using multiple methods including Kirpich, FAA, NRCS Lag, Kerby-Hathaway, and TR-55 segmental approach. Essential for hydrologic analysis and drainage design.

What This Solves

Calculates the time required for runoff to travel from the most distant point in a watershed to the outlet using various empirical methods.

Best Used When

  • You need to determine storm duration for peak flow estimation in the Rational Method
  • You are analyzing composite flow paths with sheet flow, shallow concentrated flow, and channel segments
  • You want to compare multiple Tc methods (Kirpich, NRCS, FAA, etc.) to select a representative value

Do NOT Use When

Key Assumptions

  • Flow path is a single continuous route from the most distant hydraulic point to the outlet
  • Surface roughness and slope are uniform within each flow segment type
  • Time of concentration assumes the entire watershed is contributing at equilibrium
  • Empirical equations are valid only within their original calibration ranges (area, slope, length)
  • No significant detention or storage features interrupt the flow path

Input Quality Notes

Different methods can produce widely varying results. Use methods appropriate for your land use and watershed size, and consider averaging or selecting the method that best matches local conditions and regulatory guidance.

Try a Common Scenario

Click to pre-fill the calculator with realistic values.

Calculation Method

Best for: Small agricultural watersheds with overland and channel flow. Developed from data on Tennessee watersheds.

Input Parameters

Flow Path Properties

Enter the characteristics of the longest flow path

ft

Length of the longest flow path from watershed divide to outlet

ft

Difference in elevation between the divide and outlet

Which Method Should I Use?

Kirpich

  • - Small agricultural watersheds
  • - Natural channels
  • - Simple preliminary estimates

FAA

  • - Airport drainage
  • - Parking lots
  • - Small impervious areas

NRCS Lag

  • - SCS hydrograph methods
  • - Rural watersheds
  • - When CN is known

Kerby-Hathaway

  • - Overland flow only
  • - Natural surfaces
  • - Flow length under 1000 ft

TR-55 Composite

  • - Complex flow paths with multiple segments
  • - Urban and rural watersheds
  • - Most accurate for varied terrain
  • - Required for detailed drainage design

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.

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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.