n = Manning's roughness coefficient (see Pipe Roughness),
R = Hydraulic Radius,
S = Hydraulic gradient.
The equation is in metre-per-second units but the n values are the same as those specified in the foot-per-second format of the equation (e.g. smooth concrete pipe n = 0.012 approximately). The formula is sometimes known as Strickler's formula and 1/n as the Strickler's coefficient.