An experimental program was initiated to determine the absolute roughness of three corrugated circular spiral ducts. The average absolute roughness value for these ducts was 0.85 mm (0.0028 ft). Pressure loss tests were likewise performed on a round standard spiral seam duct using Pitot-static tubes mounted at the duct centerline, and also using wall static pressure taps mounted on the duct surface at identical axial locations. The absolute roughness values obtained by these two approaches were indistinguishable. The resulting average absolute roughness for the standard spiral duct was 0.12 mm (0.0004 ft). Pressure loss predictions for corrugated and standard spiral ducts are compared.