Time of Concentration Calculator

Enter watershed length, slope, cover, and method-specific parameters to calculate time of concentration with Kirpich, FAA, NRCS Lag, Kerby-Hathaway, or TR-55 segmental methods.

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

Izzard

  • - Developed planar surfaces
  • - Short shallow sheet-flow paths
  • - When effective rainfall excess is known

TR-55 Composite

  • - Complex flow paths with multiple segments
  • - Urban and rural watersheds
  • - Most accurate for varied terrain
  • - Required for detailed drainage design
Powered by DrainageCalculators.com — free drainage & stormwater calculators