nominal pipe size of 3 inches has an actual measured
OD of 3.50 inches. A pipe with a nominal pipe size
(NPS) of 2 inches has an actual measured OD of 2.375
inches. For pipe that is 12 inches or larger, the NPS and
the actual measured OD are the same. For example, a
pipe with an NPS of 14 inches has an actual measured
OD of 14 inches. Nominal dimensions are used to
simplify the standardization of pipe fittings and pipe
taps and threading dies.
Therefore, you can use a pipe schedule to select
The wall thickness of pipe is identified by reference
either a greater or smaller pipe thickness depending on
to wall thickness schedules established by the American
the requirements of the job.
Standards Association. As an example, table 15-1
shows four schedules: 40, 80, 120, and 160. Each of
You may have seen pipe identified as STANDARD
these schedules shows a different wall thickness and,
(Std), EXTRA STRONG (XS), and DOUBLE EXTRA
therefore, a different ID for any given NPS.
STRONG (XXS). These designations also refer to wall
thickness. Figure 15-1 shows the relative wall
Assume that you have a pipe with a nominal size of
thickness of pipes having the same NPS (OD). Note
4 inches. Using table 15-1, you can see that a 4-inch
that the ID is reduced as the wall thickness is increased.
pipe can have the following ID and OD dimensions.
Pipe is manufactured in a number of different wall
Table 15-1.--Schedule Designations of Pipe Size