Pipe vs tube: how to specify dimensions
A nominal pipe size is a size designation. An outside diameter is a dimension. Write the sizing standard, wall thickness and length alongside either one so the supplier can identify the same product you need.
Start with the size system
In the ASME pipe system, NPS means nominal pipe size and DN is the corresponding nominal designation. Neither should be treated as a measured bore. For NPS 12 and smaller, the pipe outside diameter is larger than the NPS number expressed in inches. Round mechanical tube is commonly specified by outside diameter and wall thickness; always name its product standard because tube conventions vary.
| Label | Meaning | What to include in the enquiry |
|---|---|---|
| NPS / DN / NB | Nominal pipe size or bore designation | Applicable standard and nominal size; confirm the tabulated OD. |
| OD | Outside diameter | Value, units and required tolerance. |
| Wall / thickness | Thickness of the tube or pipe wall | Specified thickness and tolerance, or schedule and dimensional standard. |
| ID / bore | Inside diameter | State a bore requirement separately if it controls a fit. |
References: ASME — B36.19 stainless steel pipe.
A schedule needs a size and a standard
A schedule identifies a wall-thickness series, not one thickness for every pipe diameter. Give both the nominal size and schedule. ASME B36.10 and B36.19 cover pipe dimensions; some B36.19 schedules carry an S suffix and differ from B36.10 at particular sizes. Do not silently substitute Schedule 40 for 40S.
A dimension standard is not a material grade or a pressure rating. Keep the material specification, grade and required inspection documents on separate lines. A design engineer must establish service suitability and required wall thickness.
References: ASME — B36.19 stainless steel pipe, ASME — B36.10 welded and seamless wrought steel pipe.
Write an unambiguous pipe or tube request
- For pipe: material standard and grade; dimensional standard; nominal size; schedule or specified wall; length; quantity; welded or seamless requirement; end condition.
- For round tube: material standard and grade; OD × wall × length, with units; dimensional tolerances; quantity; delivery condition.
- For square or rectangular tube: outside width × outside height × wall × length; corner requirements where relevant; material standard and grade.
- If a machined part must fit inside, provide the required finished bore and tolerance. OD minus twice the nominal wall is only a nominal geometric calculation.
Check the same fields on every quotation
Ask the supplier to repeat the complete designation and confirm the actual offered length, tolerances, surface condition, cutting charges and weight basis. A quote for “one-inch pipe” or “25 mm tube” leaves too much unresolved. Confirm stock and the delivery location separately from the dimensional specification.
References and scope
Sources checked for this guide on 28 September 2026. Standards pages and technical summaries provide context; use the agreed standard edition and purchase specification for the order. Examples are illustrative and do not confirm product availability.
- ASME — B36.19 stainless steel pipe
Nominal sizing and the distinction between B36.19 and B36.10 schedules.
- ASME — B36.10 welded and seamless wrought steel pipe
Scope of the pipe dimensional standard. Use the applicable edition for dimensional tables.