Stainless Steel 304 Grade Mechanical Tubing

The following is an overview of how tube and pipe size is measured and handled.

OD and Nominal Pipe Size

Stainless Steel 304 Grade Mechanical Tubing is measured by the OUTSIDE DIAMETER (O.D.), specified in inches (e.g., 1.250) or fraction of an inch (eg. 1-1/4″).

Stainless Steel 304 Grade Mechanical Tubing is usually measured by NOMINAL PIPE SIZE (NPS). Although it is related to the outside diameter, it is significantly different. For example, a 1-1/2″ pipe size actually has an outside diameter of 1.9″ NOT 1.5″. The table below cross-references the nominal dimension of a PIPE with its actual outside diameter.

For sizes from 1/8-inch through 12-inch, the NPS corresponds to the nominal inside diameter, but differs somewhat from its actual inside diameter. Early pipe manufacturers made the walls in the smaller sizes much too thick, and in correcting this error in design, they took the excess from the inside to avoid changing the sizes of the companion fittings.

For sizes larger than 12-inch, NPS corresponds to the actual outside diameter. For each nominal pipe size, the outside diameter (O.D.) remains relatively constant; the variations in wall thickness affect only the inside diameter (I.D.).

The chart on the below, shows the relationship between NPS and the diameter.