Pipe Schedules Explained: NPS, Outside Diameter & ASME B36.10M Dimensions
Piping specifications frequently state dimensions like "NPS 6 Sch 40" or "DN150 Sch 80S". For newcomers and junior engineers, the relationship between Nominal Pipe Size, actual outside diameter, and schedule wall thickness can be confusing. This technical guide explains the standardization of pipe dimensions under ASME B36.10M and ASME B36.19M.
1. The Origin of "Schedule" Numbers
Historically, pipe wall thicknesses were designated simply as Standard (STD), Extra Strong (XS), and Double Extra Strong (XXS). As industrial operating pressures and temperatures increased, the American Standards Association introduced the Schedule number (Sch) based on a simplified hoop stress approximation:
Schedule Number ≈ 1000 * (P / S)
Where P is internal service pressure (psi) and S is allowable material stress (psi). While modern wall thicknesses are governed by formal tables, this historical formula explains why higher schedule numbers denote thicker walls capable of handling higher pressures.
2. Nominal Pipe Size (NPS) vs Actual Outside Diameter (OD)
A critical rule in piping engineering:
- For NPS ≤ 12" (DN300): The Nominal Pipe Size does NOT equal the actual outside diameter! For example, an NPS 2" pipe has an actual outside diameter of 2.375 inches (60.3 mm). An NPS 6" pipe has an OD of 6.625 inches (168.3 mm).
- For NPS ≥ 14" (DN350 and larger): The Nominal Pipe Size exactly equals the actual outside diameter! An NPS 14" pipe has an OD of exactly 14.00 inches (355.6 mm). An NPS 24" pipe has an OD of 24.00 inches (609.6 mm).
3. The "S" Suffix for Stainless Steel (ASME B36.19M)
Carbon steel pipe dimensions are standardized under ASME B36.10M. Stainless steel and nickel alloy pipe dimensions are governed by ASME B36.19M and carry an "S" suffix (e.g. Sch 5S, Sch 10S, Sch 40S, Sch 80S). In small sizes, Sch 40 and Sch 40S share identical wall thicknesses, but in larger sizes, ASME B36.19M designates thinner walls for stainless steel to optimize material costs while leveraging stainless steel's superior corrosion resistance.
4. Reference Dimension Table (NPS 1/2" to 12")
| NPS (Inches) | DN (Metric) | Outside Diameter (OD mm) | Sch 40 / STD Wall (mm) | Sch 80 / XS Wall (mm) | Sch 160 Wall (mm) |
|---|---|---|---|---|---|
| 1/2" | DN15 | 21.3 mm | 2.77 mm | 3.73 mm | 4.78 mm |
| 1" | DN25 | 33.4 mm | 3.38 mm | 4.55 mm | 6.35 mm |
| 2" | DN50 | 60.3 mm | 3.91 mm | 5.54 mm | 8.74 mm |
| 4" | DN100 | 114.3 mm | 6.02 mm | 8.56 mm | 13.49 mm |
| 6" | DN150 | 168.3 mm | 7.11 mm | 10.97 mm | 18.26 mm |
| 8" | DN200 | 219.1 mm | 8.18 mm | 12.70 mm | 23.01 mm |
| 10" | DN250 | 273.0 mm | 9.27 mm | 15.09 mm | 28.58 mm |
| 12" | DN300 | 323.8 mm | 10.31 mm | 17.48 mm | 33.32 mm |