This guide supports procurement discussion and does not replace the approved design code, materials-engineering review or project specification. Verify the current edition of every referenced standard.
Schedule is a wall designation
A schedule identifies a standardised wall-thickness series for pipe. It is not itself a pressure rating, and the same schedule number does not represent one constant wall thickness across all nominal sizes.
Nominal pipe size identifies a size system; the actual outside diameter and wall are taken from the applicable dimensional standard. Stainless and wrought-steel dimensional tables must be consulted for the exact combination.
- State NPS or DN, schedule or exact wall, and dimensional standard.
- Do not calculate pressure capability from schedule alone.
- Confirm bore requirements where flow area or mating components are critical.
How to specify pipe dimensions
A complete dimensional line typically includes nominal size, outside diameter where relevant, schedule or wall, length and end preparation. Add tolerances or minimum-wall requirements when the project requires them.
The pressure design must also consider material allowable stress, temperature, corrosion allowance, manufacturing factors and the governing design code.
Common RFQ fields
| Field | What it controls | Buyer check |
|---|---|---|
| NPS / DN | Nominal size series | Use one clear designation |
| Schedule / wall | Nominal wall thickness | Name the dimensional standard |
| Length | Random, fixed or cut length | State tolerance if critical |
| Ends | Plain, bevelled or threaded | Coordinate with joining method |
| Design conditions | Engineering verification | Do not replace with schedule alone |
Common buyer questions.
Does Schedule 40 have the same wall in every size?+
No. The wall thickness varies by nominal size.
Is schedule the same as pressure class?+
No. Pipe schedule describes wall series; flange pressure class is a different designation.
Can exact millimetre wall be specified instead?+
Yes where permitted by the standard and design, but availability and tolerances must be reviewed.
Standards and technical sources.
Links are provided for source context. Always use the edition required by the contract and approved project documents.
ASME B36.10 — Welded and Seamless Wrought Steel PipeASME B36.19 — Stainless Steel Pipe