Time of Concentration Calculator

Time of Concentration

Input Data:
Slope = ft/ft
Length of overland flow = ft
Watershed area = sq. mi.
Retardance coefficient =
Rainfall intensity = in/hr
Rational coefficient =
Retardance roughness =
Mannings overland roughness =
Izzards formula = min.
Kerbys equation = min.
Kirpichs equation = min.
Kinematic equation = min.
Bransby Williams equation = min.
FAA equation = min.


Izzards formula


K = 0.0007i + Cr


Kerbys equation

  tc = c (Ln/i1/2)0.467

Kirpichs equation

tc = 0.0078 

Kinematic equation

 L0.6 n0.6
tc = 0.0078 

Bransby Williams equation

tc = 21.3 
 5280 A0.1 S0.2

FAA equation

 1.8 (1.1 - C) L0.5
tc =
C = the dimensionless runoff coefficient
L = the distance traveled, in feet, and
S = the slope, in percent


Retardance coefficient

Very smooth asphalt:0.007
Tar and sand pavement:0.0075
Tar and gravel pavement:0.017
Closely clipped sod:0.046
Dense bluegrass:0.060

Rainfall intensity

Time of concentration is calculated from rainfall intensity iteratively using an IDF curve. Enter the trial rainfall intensity in inches per hour. Use the calculated tc to find a new intensity from the IDF curve.

Rational coefficient

Downtown Business:0.70 - 0.95
Single Family Res:0.30 - 0.50
Asphalt/Concrete:0.70 - 0.95
Sandy Soil Lawn:0.05 - 0.20
Heavy Soil Lawn:0.13 - 0.35
Brick:0.70 - 0.85

Retardance roughness

Smooth Pavement:0.02
Poor grass, bare sod:0.30
Average grass:0.40
Dense grass:0.80

Mannings overland roughness

Bare sand:0.010
Natural range:0.080
Bluegrass sod:0.450
Short prairie:0.150