Pipe fitting size information can be confusing because buyers often see several terms in one specification: NPS, DN, outside diameter, wall thickness and schedule. Understanding these terms helps purchasing teams choose elbows, tees, reducers and flanges that fit the pipe correctly and meet pressure requirements.
What is NPS?
NPS stands for Nominal Pipe Size. It is a North American pipe size designation used to identify pipe and fitting sizes. NPS is not always the same as the actual outside diameter, especially for smaller pipe sizes. When ordering industrial pipe fittings, NPS should be used together with wall thickness or schedule.
What is DN?
DN means Diameter Nominal and is commonly used in metric specifications. DN and NPS are often paired in international projects. For example, NPS 2 is commonly associated with DN 50. Buyers should always confirm the project standard rather than relying only on approximate conversions.
What does pipe schedule mean?
Pipe schedule describes wall thickness. Common schedules include SCH 10, SCH 40, SCH 80 and SCH 160. A higher schedule generally means a thicker wall and higher pressure capability, depending on material and design code. Fittings should match the pipe schedule or the approved project specification.
Why wall thickness matters for fittings
Wall thickness affects weld preparation, pressure performance, weight and cost. If the fitting wall thickness does not match the pipe, the weld joint may require additional preparation or may not meet project requirements. This is especially important for butt weld elbows, tees and reducers under ASME B16.9.
Common information needed for a size inquiry
- Fitting type: elbow, tee, reducer, cap or flange
- Nominal size: NPS or DN
- Wall thickness or schedule
- Material grade
- Applicable standard
- Quantity and inspection requirements
Example specification
A complete inquiry might be: ASME B16.9 90 degree long radius elbow, NPS 4, SCH 40, ASTM A234 WPB, butt weld ends. For stainless steel, an example could be: 316L stainless steel concentric reducer, DN 100 x DN 50, SCH 10S.
Related products
Use this sizing guide together with our pipe elbows, pipe tees, pipe reducers and pipe flanges product pages.
FAQ
Is NPS the same as outside diameter?
No. NPS is a nominal size system. The actual outside diameter should be checked from the relevant pipe standard.
Should fittings match pipe schedule?
In most welded piping systems, fitting wall thickness should match the pipe schedule or follow the project specification.
Need help confirming pipe fitting dimensions? Send your size, material and standard requirements to PipingFitting.com.