Three indirect methods for characterizing the drag of an arbitrarily rough surface on a flat plate are derived or rederived from the similarity laws of turbulent shear flows. These are:
the well-known procedure using displacement thickness;
the probably not-so-well-known procedure using total drag; and
a proposed new procedure using only the freestream velocity and a local shear stress. The classical indirect procedure for pipe flow is also derived to show a commonality with the flat-plate methods.