For flows at Mach numbers much less than unity the fully-developed wall shear stress in a circular pipe may be determined directly from a knowledge of the pipe radius, R, and the measured gradient in static pressure along the pipe, dp/dx. At higher subsonic Mach numbers it is conventional to apply a one-dimensional compressibility correction to make allowance for the acceleration of the fluid arising from the progressive decrease in density along the pipe. The correction procedure is to determine a “frictional” pressure gradient, dpf/dx, associated with wall shear stresses alone.