The rational C coefficient is the parameter that is the most open to engineering judgment. It is a unitless number between 0.0 and 1.0 that relates the rate of rainfall over a catchment to the rate of discharge from that catchment. A value of 0.0 implies that none of the rainfall is discharged from the catchment, while a value of 1.0 implies that all of the rainfall is immediately discharged from the catchment.

The coefficient is highly dependent on land use and slope approaching 1.0 for impervious ground covers, such as pavement. For some common C values for various types of land cover and slope, see the Engineer's Reference section at the end of this appendix.