Schedule 40 Stainless Steel Pipe: Dimensions and 40S Differences
Look up stainless Schedule 40S pipe OD, wall and ID, check where Schedule 40 differs, and specify grade, manufacturing method and ends separately.
A 2-inch Schedule 40 stainless steel pipe has a 2.375-inch outside diameter and a 0.154-inch nominal wall—not a 2-inch OD. Its calculated nominal inside diameter is 2.067 inches. At this size, Schedule 40 and stainless Schedule 40S have the same dimensions. At NPS 12, their walls differ. See the Quest 40S chart and Unified Alloys schedule comparison.
The schedule identifies a wall-thickness series for a given nominal pipe size (NPS). It does not identify the stainless grade, surface finish or allowable working pressure.
Stainless Schedule 40S size chart
These are nominal dimensions for NPS 1/2 through 12. Schedule 40 has the same OD and wall as 40S for the listed sizes through NPS 10; use the comparison below for NPS 12.
The table uses inch OD and wall dimensions from Tioga’s pipe chart, with metric OD values as published in the Quest stainless 40S chart. ID is calculated from the inch dimensions as OD − 2 × wall.
| NPS | OD (in.) | OD (mm) | 40S wall (in.) | 40S wall (mm) | Nominal ID (in.) |
|---|---|---|---|---|---|
| 1/2 | 0.840 | 21.3 | 0.109 | 2.77 | 0.622 |
| 3/4 | 1.050 | 26.7 | 0.113 | 2.87 | 0.824 |
| 1 | 1.315 | 33.4 | 0.133 | 3.38 | 1.049 |
| 1-1/4 | 1.660 | 42.2 | 0.140 | 3.56 | 1.380 |
| 1-1/2 | 1.900 | 48.3 | 0.145 | 3.68 | 1.610 |
| 2 | 2.375 | 60.3 | 0.154 | 3.91 | 2.067 |
| 2-1/2 | 2.875 | 73.0 | 0.203 | 5.16 | 2.469 |
| 3 | 3.500 | 88.9 | 0.216 | 5.49 | 3.068 |
| 3-1/2 | 4.000 | 101.6 | 0.226 | 5.74 | 3.548 |
| 4 | 4.500 | 114.3 | 0.237 | 6.02 | 4.026 |
| 5 | 5.563 | 141.3 | 0.258 | 6.55 | 5.047 |
| 6 | 6.625 | 168.3 | 0.280 | 7.11 | 6.065 |
| 8 | 8.625 | 219.1 | 0.322 | 8.18 | 7.981 |
| 10 | 10.750 | 273.1 | 0.365 | 9.27 | 10.020 |
| 12 | 12.750 | 323.9 | 0.375 | 9.53 | 12.000 |
Nominal ID is a geometric reference, not a guaranteed minimum bore. Manufacturing tolerances affect actual dimensions. Inch and metric values are shown as published; for an order, use the specified unit system rather than mixing rounded values. ASTM A312/A312M treats the two unit systems independently.
Schedule 40 versus 40S
The S suffix distinguishes ASME B36.19 stainless-pipe dimensions from ASME B36.10 dimensions. It does not mean seamless: B36.19 covers both welded and seamless pipe. ASME also notes that B36.10 wall thicknesses are commercially available in stainless steel. See ASME’s stainless-pipe standard description.
Selected larger sizes show why “40” and “40S” should not be treated as interchangeable:
| NPS | OD (in.) | Schedule 40S wall (in. / mm) | Schedule 40 wall (in. / mm) |
|---|---|---|---|
| 12 | 12.750 | 0.375 / 9.53 | 0.406 / 10.31 |
| 14 | 14.000 | 0.375 / 9.53 | 0.438 / 11.13 |
| 16 | 16.000 | 0.375 / 9.53 | 0.500 / 12.70 |
Sources: Unified Alloys and Tioga.
At NPS 12, the calculated bore is 12.000 inches for 40S versus 11.938 inches for 40. The outside diameter remains unchanged. Put the dimensional standard, schedule and nominal wall on the purchase description to remove ambiguity.
For a thinner-wall comparison, see Schedule 10 pipe thickness by NPS and type.
What to specify before buying
Keep these requirements separate:
- Dimensions: NPS, dimensional standard, schedule and nominal wall.
- Material: exact stainless grade—not just “stainless.”
- Product specification: ASTM A312/A312M where appropriate. It covers seamless, straight-seam welded and heavily cold-worked welded austenitic stainless pipe for high-temperature and general corrosive service; it is not a schedule designation. See the ASTM scope.
- Manufacturing and delivery: welded or seamless, length, quantity, end preparation, required surface condition and material documentation.
For example, a dimensionally clear line is: NPS 2 stainless pipe, ASME B36.19, Schedule 40S, nominal wall 0.154 in. Add the approved grade and remaining requirements before ordering.
Do not substitute 2-inch-OD tube for NPS 2 pipe: the pipe OD is 2.375 inches. ASME explicitly distinguishes pipe’s nominal sizing from tube sizing by actual OD. Check the connection manufacturer’s dimensional requirements, not just the size label. See ASME’s sizing explanation.
Schedule 40 is not a pressure rating
There is no single safe working pressure for all Schedule 40 stainless pipe. A published theoretical burst value is not an allowable operating pressure: for example, Unified Alloys’ burst chart assumes a specified ultimate strength and ambient temperature. See its chart calculation notes.
For pressure service, verify the material, design temperature, wall allowances, joints and component limits under the applicable piping code. Where it applies, ASME B31.3 covers materials, components, design, fabrication, inspection and testing—not merely pipe dimensions.