NPS vs Actual Pipe Diameter
NPS vs Actual Outside Diameter
Why NPS Does Not Equal Actual Pipe Diameter
NPS, or Nominal Pipe Size, is a standardized designation used to identify pipe size. It is not a direct measurement of the pipe's outside diameter.
For example, NPS 2 pipe has a nominal size of 2 inches, but its standard outside diameter is 2.375 inches, or 60.33 mm.
The outside diameter remains the same for a given NPS across different pipe schedules. Schedule changes normally affect wall thickness, inside diameter, and pipe weight rather than the standard outside diameter.
Reference standard: ASME B36.10-2022.
Quick Examples
- NPS 2 → 2.375 in OD → 60.33 mm OD
- NPS 3 → 3.5 in OD → 88.90 mm OD
- NPS 4 → 4.5 in OD → 114.30 mm OD
- NPS 6 → 6.625 in OD → 168.28 mm OD
NPS, DN, and Actual Diameter
NPS and DN are nominal size systems, while outside diameter is an actual physical dimension. For example, NPS 2 corresponds to DN 50, but the actual outside diameter is 2.375 inches rather than exactly 2 inches.
See the NPS to DN conversion chart for nominal size conversions, or browse the steel pipe sizes chart for the complete NPS reference.
Related Pipe References
FAQ
Is NPS the actual pipe diameter?
No. NPS is a nominal pipe size designation. The actual outside diameter depends on the standardized NPS size.
What is the actual outside diameter of NPS 2 pipe?
NPS 2 pipe has a standard outside diameter of 2.375 inches, or 60.33 mm.
Does Schedule 40 change the outside diameter?
No. For a given NPS, Schedule 40 and Schedule 80 normally use the same standard outside diameter. The schedule changes wall thickness, inside diameter, and weight.
What is the difference between NPS and DN?
NPS is the Nominal Pipe Size designation used primarily in inch-based standards, while DN is the nominal diameter designation used in metric-based systems. They are related nominal size systems, not direct measurements of actual pipe diameter.