Overview
Nominal Pipe Size, or NPS, is a standard size designation used to identify compatible pipe, fittings, valves, and flanges. It is not a direct measurement of outside diameter or inside diameter for every size. DN is the corresponding metric nominal designation used in many international documents.
IPS, or Iron Pipe Size, is an older sizing term that influenced modern NPS usage. In current technical documents, NPS or DN should be paired with the applicable dimensional standard and wall schedule or thickness.
Why It Matters for Pipe Components
Confusing nominal size with measured diameter causes wrong flange bores, clamps, fittings, threads, and replacement parts. The OD may remain constant across several schedules while the ID changes with wall thickness.
NPS and DN are designations rather than exact unit conversions. A DN value should not be treated as a millimetre measurement of the bore.
Product families use nominal size differently. Pipe threads, tube fittings, plastic pipe, hose, and structural tubing can use other sizing conventions.
Methods, Parameters, or Process
Read a pipe size description from designation to actual dimensions.
| Term | Meaning | What it does not mean |
|---|---|---|
| NPS | Nominal Pipe Size designation | Exact OD or ID for every size |
| DN | Metric nominal designation | Exact bore in millimetres |
| Schedule | Wall-thickness series for applicable pipe standard | Independent pressure rating |
| OD | Actual outside diameter basis | Nominal size by itself |
| ID | Actual or calculated inside diameter | Constant value across schedules |
| IPS | Legacy iron-pipe sizing terminology | A complete modern specification |
Supplier and Inspection Checklist
- State NPS or DN and the pipe dimensional standard.
- Add schedule or nominal wall thickness.
- Use actual OD for clamps, supports, and special interfaces.
- Use bore or schedule for weld-neck flanges.
- Check reducing fitting sizes at both ends.
- Do not convert DN directly into measured millimetres.
- Confirm whether the product is pipe, tube, hose, or conduit.
- Use controlled dimensional tables for design and inspection.
Application to Pipe Fittings, Valves, and Flanges
Elbows, tees, reducers, and caps are ordered by nominal pipe size plus wall and material. Flanges add class or PN, facing, type, and bore. Valves add end standard, bore, trim, and pressure class.
For replacement parts, measure actual dimensions but identify the nominal system before selecting a catalogue product.
FAQ
What does nominal pipe size mean?
It is a standard designation for compatible piping components, not always a direct measured diameter.
Is NPS the same as OD?
Not generally. Use the applicable pipe standard for actual OD.
What is IPS vs NPS?
IPS is legacy iron-pipe sizing terminology; NPS is the widely used modern nominal designation.
Is DN80 exactly 80 mm ID?
No. DN80 is a nominal designation. Actual OD and ID depend on standard and wall.
Does schedule change OD?
For common steel pipe within a nominal size, schedule mainly changes wall and ID, but always verify the applicable standard.
Next Steps
Write pipe requirements as nominal size plus standard, schedule or wall, material, length, ends, and tests. Add actual OD or bore when interfacing with custom equipment.
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