What does pipe schedule mean?
Pipe schedule is a designation for wall thickness. Nominal pipe size fixes the outside diameter, and the schedule number fixes how thick the wall is, so the bore shrinks as the schedule rises. A 6 inch SCH 40 and a 6 inch SCH 80 pipe share an outside diameter of 168.3 mm but have walls of 7.11 mm and 10.97 mm.
Key Takeaways
- Outside diameter is set by nominal pipe size and does not change with schedule — only the wall thickness and bore change.
- A higher schedule number means a thicker wall, a smaller bore and a higher pressure rating.
- SCH 40 is the general-service default; SCH 80 is common on higher-pressure and small-bore lines.
- The S-series schedules (10S, 40S, 80S) apply to stainless steel and are not identical to the carbon series at every size.
- Above NPS 12 inch, nominal size and outside diameter in inches become the same number — NPS 14 inch pipe has a 355.6 mm OD.
- XXS is a legacy designation carried over from double extra strong and does not follow the numeric schedule progression.
The chart
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| NPS | OD (mm) | SCH 10 | SCH 20 | SCH 40 | SCH 80 | SCH 160 | XXS |
|---|---|---|---|---|---|---|---|
| 1/2" | 21.3 | 2.11 | — | 2.77 | 3.73 | 4.78 | 7.47 |
| 3/4" | 26.7 | 2.11 | — | 2.87 | 3.91 | 5.56 | 7.82 |
| 1" | 33.4 | 2.77 | — | 3.38 | 4.55 | 6.35 | 9.09 |
| 1-1/2" | 48.3 | 2.77 | — | 3.68 | 5.08 | 7.14 | 10.15 |
| 2" | 60.3 | 2.77 | — | 3.91 | 5.54 | 8.74 | 11.07 |
| 3" | 88.9 | 3.05 | — | 5.49 | 7.62 | 11.13 | 15.24 |
| 4" | 114.3 | 3.05 | — | 6.02 | 8.56 | 13.49 | 17.12 |
| 6" | 168.3 | 3.40 | — | 7.11 | 10.97 | 18.26 | 21.95 |
| 8" | 219.1 | 3.76 | 6.35 | 8.18 | 12.70 | 23.01 | 22.23 |
| 10" | 273.0 | 4.19 | 6.35 | 9.27 | 15.09 | 28.58 | 25.40 |
| 12" | 323.8 | 4.57 | 6.35 | 10.31 | 17.48 | 33.32 | 25.40 |
| 14" | 355.6 | 6.35 | 7.92 | 11.13 | 19.05 | 35.71 | — |
| 16" | 406.4 | 6.35 | 7.92 | 12.70 | 21.44 | 40.49 | — |
| 18" | 457.0 | 6.35 | 7.92 | 14.27 | 23.83 | 45.24 | — |
| 20" | 508.0 | 6.35 | 9.53 | 15.09 | 26.19 | 50.01 | — |
| 24" | 610.0 | 6.35 | 9.53 | 17.48 | 30.96 | 59.54 | — |
How to read the chart
Find the nominal pipe size in the first column and read the outside diameter beside it. That figure is fixed — every schedule at that size shares it, which is what allows a SCH 40 and a SCH 80 pipe to fit the same flange and the same fitting.
Then read across to the schedule column for the wall thickness. Inside diameter is the outside diameter minus twice the wall. For 6 inch SCH 40: 168.3 − (2 × 7.11) = 154.08 mm bore.
Two quirks are worth knowing. Below NPS 14 inch, the nominal size and the outside diameter are unrelated numbers — 2 inch pipe has a 60.3 mm outside diameter, not 50.8 mm. From NPS 14 inch upward, nominal size and outside diameter in inches are the same number.
Choosing a schedule
Schedule is selected on pressure, corrosion allowance and mechanical robustness, and the three do not always point the same way.
- SCH 10 and 10S — low-pressure and utility service, and stainless lines where no corrosion allowance is required.
- SCH 40 — the general-service default across most process and utility piping.
- SCH 80 — higher pressure, and standard on small-bore lines where mechanical damage is the real risk rather than pressure.
- SCH 160 and XXS — high-pressure service, steam headers and positions carrying a large corrosion allowance.
On corrosive service the corrosion allowance is added to the pressure-derived thickness, which frequently pushes the selection a full schedule higher than the pressure calculation alone would suggest.
Matching fittings and flanges to the schedule
A buttweld fitting is specified to match the schedule of the pipe it joins, because the weld preparation has to line up on both bore and wall. Ordering a fitting by nominal size alone, without the schedule, is the most frequent cause of material arriving that cannot be welded into the run without a transition.
Weld neck flanges are bored to the schedule for the same reason. When ordering, state the schedule alongside the size and class: “6 inch, Class 300, RF weld neck, bored to SCH 40”. See the ASME B16.5 flanges page for the flange side, and ASTM A234 WPB for buttweld fittings.
Frequently Asked Questions
SCH 40 is a wall thickness designation. At 6 inch nominal size it means a 7.11 mm wall on a 168.3 mm outside diameter. The schedule number sets the wall, not the outside diameter.
No. Outside diameter is fixed by nominal pipe size. Raising the schedule thickens the wall inward, so the bore shrinks while the outside diameter stays the same.
The S series applies to stainless steel under ASME B36.19M. At many sizes SCH 40 and 40S are identical, but they diverge at others, so the S designation should be carried through to the purchase order rather than dropped.
XXS stands for double extra strong, a legacy designation predating the numeric schedule system. It does not sit at a fixed point in the schedule progression, and at some sizes SCH 160 is actually thicker than XXS.
Yes. The chart is published ungated, with no sign-up and no email address required, and can be printed or referenced freely.