There are two common methods for designating pipe outside diameter (OD). The North American method is called NPS ("Nominal Pipe Size") and is based on inches. The European version is called DN ("Diameter Nominal" / "Nominal Diameter") and is based on millimetres.

Designating the outside diameter allows pipes of the same size to be fit together no matter what the wall thickness.

For pipe sizes less than NPS 14 inch (DN 350),

Both methods give a nominal value for the OD that is rounded off and is not the same as the actual OD.

For pipe sizes of NPS 14 inch (DN 350) and greater

The NPS size is the actual diameter in inches and the DN size is equal to NPS times 25 (not 25.4) rounded to a convenient multiple of 50.