Is Pipe Measured I.D. or O.D.
First, this question cannot be answered with a blanket statement; the determination depends on the specific standards the piping adheres to, as well as the type of pipe involved.
Under API standards, pipe dimensions are defined by the Outer Diameter (O.D.) and the Schedule; consequently, the Inner Diameter (I.D.) varies as the wall thickness changes.
Under EN and ASTM standards, pipe dimensions can be determined using either the O.D. plus wall thickness, or the I.D. plus wall thickness.
In addition to classification based on standards, pipe dimensions can also be determined based on their intended application.
The term “Pipe” generally refers to larger-sized conduits, where specific dimensions are defined by the Nominal Pipe Size (NPS) and the Schedule (SCH).
The term “Tube” typically refers to precision tubing, where the actual dimensions are generally specified by the O.D. plus wall thickness, or the I.D. plus wall thickness.
Please note that O.D. and I.D. represent actual physical dimensions.
NPS, however, does not represent an actual physical dimension; rather, it is a nominal size designation, and one must consult a dimension chart to determine the corresponding actual dimensions.
