Pipes with large roughnesses can occur due to erosion, corrosion, and scale. These roughnesses can be much larger than are accounted for in the Moody diagram. Experimental tests have shown that laminar flow of non-Newtonian fluids in pipes with large roughnesses can be accurately predicted by using a constricted diameter, where the diameter measurement is taken as the typical minimum of the rough pipe. Turbulent flows of non-Newtonian fluids are less accurately predicted by available methods and it is suggested that test work be performed if it is required to account for large pipe roughness.