There are two methods commonly used to name the size of a pipe, namely:
- NPS: Nominal Pipe Size, widely used in North America, with inch units.
- DN: Nominal Diameter used by countries in mainland Europe, with millimeters units.
DN and NPS show the size of diameter. Therefore need to know how to find the naming of pipe thickness sizes which is commonly referred to as Schedule. Schedule is the naming of pipe sizes that show the size of pipe wall thickness.
For countries which always use millimeter as their units will commonly confuse if use NPS (Nominal Pipe Size) to naming size of pipe. The size of pipe in NPS is not always measure and show outside diameter (OD) actually. The difference between NPS and OD is starting from pipe sizes NPS ¼ "up to pipe size NPS 12". But for pipe size NPS above 12 inches show size of outside diameter actually or in other word, size NPS above 12 inch is same with size in millimeter.
In some literatures explained that the difference is more due at the beginning of pipe manufacturing in 1930s are based on inside diameter pipe with 1/16" wall thickness, so the size becomes larger outer diameter 1/8". Over time, technology of pipe manufacturing is advancing so pipe manufacturer can produce pipe with smaller wall thickness but can maintain outside diameter pipe size.