#1 Steel tubes

Steel tube is generally used for structural purposes and the OD is an important and exact number. Steel tube size is specified by OD and the wall thickness, and the measured OD and stated OD are generally within very close tolerances of each other. Steel tube is usually more expensive than the steel pipe due to tighter manufacturing tolerances. Interestingly, while the stated and measured OD’s of steel tubes are almost exact most of the time, steel tube generally has a measured OD that is 1/8” larger than stated OD.

#2 Steel pipes

Steel pipes are categorized as tubular vessels used in pipeline and piping systems, and commonly transport gases or fluids. They are specified by “Nominal Pipe Size” (NPS) and Schedule (wall thickness). NPS is a size standard established by the American National Standards Institute (ANSI), and should NOT be confused with the various thread standards such as NPT and NPSC.

The manufacturing of Nominal Pipe Sizes from 1/8” to 12” is based on a standardized nominal outside diameter (OD) that is different from the measured OD. NPS pipe 14” and up have measured OD’s that correspond to the nominal size.