JavaScript must be enabled in order to use this site.Please enable JavaScript in your browser and refresh the page. SCS Lag Note: There is a factor of 0.6 built into this equation (in the constant 0.0000877) to convert this equation from a lag time to a time of concentration. Tc = Time of concentration (hr.) Lf = Flow length (ft) n = Manning’s n Sf = Slope (ft/ft) Parent topic: Time of Concentration