Explaining Dimensional Differences in Pipe Sizes, Schedules & Material

The different names used in pipe sizes

Nominal Pipe Size (NPS)
NPS is an abbreviation of Nominal Pipe Size, which is a term used as a guideline number defining the diameter of the pipe.
Nominal pipe size refers to only the outside diameter (OD) of a pipe making it somewhat vague. For example, when we say pipe size is 2 NPS, it refers to all the pipes having 2.375-inch (or 60.3 mm) as outside diameter irrespective of wall thickness and inside diameter (ID).
Nominal Bore (NB)
is the European designation equivalent to NPS, in which sizes are measured in millimetres. NB (nominal bore) is also frequently used interchangeably with NPS.   
Diameter Nominal (DN) 
This is the international and European metric method used to describe NPS (or NB).DN pipe sizes numbers are therefore different to NPS. For example, the metric equivalent of an NPS 6 pipe (6”) is DN 150. (To convert from NPS 6 to DIN simply multiply the 6 x 25=150. (150mm)
Outside Diameter (OD)
OD is the outside diameter of the pipe and is fixed for a given size.
The NPS is very loosely related to the inside diameter in inches, but only for NPS 1/8 to NPS 12. For NPS 14 and larger, the NPS is equal to the outside diameter (OD) in inches. For a given NPS, the OD stays constant and the wall thickness increases with larger SCH. For a given SCH, the OD increases with increasing NPS while the wall thickness increases or stays constant.

pipe sizes

What are pipe schedules?

Steel pipe schedules are a way to describe the wall thickness of the pipe. This is a critical parameter as it is directly related to the strength of the pipe and the suitability for specific applications. A pipe schedule is a dimensionless number and is calculated based on the design formula for wall thickness, given the design pressure and allowable stress.
The most common schedule numbers are 40 and 80. As the schedule number increases, the wall thickness of the pipe increases. The schedule number is roughly calculated as per formula below:
Schedule = 1000 x (P/S)
P = Internal service pressure of pipe (psig)
S = Ultimate tensile strength of the pipe material (psi)

pipe sizes and schedules

How Pipe material & type affects the outside diameter

In the field of engineering, there are many and varied materials & types of pipes used, each with their own unique credentials, and often with their own sizing. This results in some confusion in the industry around the outside diameter of piping, and the sizing of required pipe supports. We have outlined the dimensions of 10 of the more common pipe materials in the chart below.
As a manufacturer of pipe supports we can cater for all sizes, in all materials.

pipe sizes and materials conversion chart