Pipe Sizes Explained

Pipe Sizing History

Defining the size of a pipe may at first seem rather simplistic, and initially it was. About 100 years ago, pipes were sized according to their iron pipe size (IPS). If a pipe had an internal diameter of approximately 6 inches, it was classed as an IPS-6 size pipe. The wall thickness of all pipes was standardised (standard (STD)), as was -consequently- their outside diameter.

The IPS system continued to develop as service pressure and temperature ranges increased. To cater for these increased service demands, pipe walls were made progressively thicker and categorised as extra strong (XS)  or double extra strong (XXS). It became obvious to engineers working with the IPS system that continuing to triple extra strong (XXXS) or quadruple extra strong (XXXXS) was illogical, especially since advancements in material science (newly developed alloys etc.) made thinner pipe walls possible. To solve the issues piping engineers were encountering, a nominal pipe size (NPS) system was developed to better suit the constantly changing piping industry.

Nominal Pipe Size (NPS)

Pipes in the nominal pipe size (NPS) system are sized using a dimensionless designation. The terms nominal bore (NB) and nominal pipe size can be used interchangeably i.e. they mean the same thing. The term ‘national pipe size (NPS)’ is sometimes used, but this term is completely incorrect.

A dimensionless designation simply means that there are no units of measurement indicated on an NPS pipe (no inches etc.). However, the NPS system is associated with the imperial measurement system. A pipe stamped as NPS 3 is actually a pipe with an outside diameter (OD) measuring 3.5 inches, not 3 inches. This unusual system developed because pipes were originally manufactured to maintain the internal diameter (ID) of their predecessors, based on the standard wall (STD) thickness used at the time. As time passed, the internal diameter could be varied as required using the new pipe thickness system (schedule system), but the NPS size designations were maintained.

NPS to Outside Diameter Table

Diameter Nominal (DN)

The alternative to the imperial NPS is the metric version diameter nominal (DN), developed by the International Standards Organisation (ISO). As with the NPS system, the DN system is dimensionless, although it is based on the millimetre (mm) unit. A DN 80 pipe relates to a pipe having an outside diameter of 88.90 mm, not 80.00 mm. The DN system was created after the NPS system and conversion tables were developed to compare the two pipe size systems. The below table shows NPS values along with their DN and OD counterparts.

NPS, DN, OD, Comparison Table 1

* Source: https://en.wikipedia.org/wiki/Nominal_Pipe_Sizef

Larget Pipe Sizes

The NPS pipe size designations are counter intuitive until pipe sizes reach greater than 12 inches (DN 300). Pipe sizes exceeding 12 inches have a logical relationship with their NPS designation. For example, an NPS 14 pipe has an outside diameter of 14 inches and an NPS 16 pipe has an outside diameter of 16 inches.

NPS Size Measurement

Every pipe designation greater than NPS 12 maintains this relationship i.e. the NPS designation indicates the outside diameter of a pipe in inches. Further examples can be seen in the below table

NPS, DN, OD, Comparison Table 2

* Source: https://en.wikipedia.org/wiki/Nominal_Pipe_Size

Note that the DN pipe size designation system retains a counter-intuitive pattern even after the NPS size exceeds NPS 12. The DN designation roughly equals a pipe’s outside diameter in millimetres. A pipe’s actual outside diameter measured in millimetres always exceeds its DN designation. For example, a DN 300 pipe has an outside diameter in millimetres greater than 300 millimetres (it is actually 323 mm).

Additional Resources

https://en.wikipedia.org/wiki/Nominal_Pipe_Size

https://www.trupply.com/pages/socket-weld-flange

https://hardhatengineer.com/pipe-class-piping-specifications-pipeend/pipe-schedule-chart-nominal-pipe-sizes

https://www.theprocesspiping.com/nominal-pipe-size-and-schedule