Pipe Schedule Chart Explained: SCH 40, SCH 80, STD and XS
What pipe schedules mean, how nominal pipe size relates to outside diameter, wall thickness tables for common sizes, and why SCH 40 is not one fixed thickness.
Saudi Steel Work Editorial Team · · 3 min read
Short answer: A pipe schedule (SCH) is a number from ASME B36.10M that defines wall thickness for each nominal pipe size (NPS). The outside diameter is fixed for each NPS, and higher schedules mean thicker walls. Wall thickness for a given schedule changes with pipe size, so SCH 40 on a 2-inch pipe is thinner than SCH 40 on an 8-inch pipe.
Pipe dimensions confuse many buyers because nominal size, outside diameter and schedule all interact. Here is a practical explanation with the sizes you'll meet most often.
Nominal pipe size (NPS) and DN
- NPS is the North American designation in inches (e.g. 4").
- DN is the metric equivalent (e.g. DN 100).
- For each NPS, the outside diameter (OD) is fixed regardless of schedule. Changing schedule changes the bore.
Common sizes (ASME B36.10M)
| NPS | DN | OD (mm) | SCH 40 / STD wall (mm) | SCH 80 / XS wall (mm) |
|---|---|---|---|---|
| ½" | 15 | 21.3 | 2.77 | 3.73 |
| ¾" | 20 | 26.7 | 2.87 | 3.91 |
| 1" | 25 | 33.4 | 3.38 | 4.55 |
| 1½" | 40 | 48.3 | 3.68 | 5.08 |
| 2" | 50 | 60.3 | 3.91 | 5.54 |
| 3" | 80 | 88.9 | 5.49 | 7.62 |
| 4" | 100 | 114.3 | 6.02 | 8.56 |
| 6" | 150 | 168.3 | 7.11 | 10.97 |
| 8" | 200 | 219.1 | 8.18 | 12.70 |
| 10" | 250 | 273.0 | 9.27 | 12.70 (XS) |
| 12" | 300 | 323.8 | 9.53 (STD) | 12.70 (XS) |
For 10" and 12", SCH 80 is heavier than XS; always confirm the exact designation.
Schedule vs weight class
- STD (standard), XS (extra strong) and XXS (double extra strong) are weight classes.
- SCH 10, 20, 40, 80, 160 are schedules.
- They coincide for many small sizes but diverge for larger ones.
Why it matters
- Pressure rating depends on wall thickness, material and temperature — not schedule alone.
- Weight and cost depend on wall thickness. See how to calculate steel pipe weight.
- Fittings and flanges must match the schedule (bore and weld preparation).
Ordering tip
Specify NPS and schedule and standard, for example: "4" SCH 40, ASTM A106 Gr.B, seamless, bevelled ends, 6 m". Adding wall thickness in mm removes any doubt.
See ERW vs seamless and our steel pipes range.