Steel Pipe Size Chart: Nominal Bore, OD, WT & Schedule Explained
If you work with steel pipes, you have probably stared at a pipe size chart and felt confused. Nominal Bore (NB), Outside Diameter (OD), Wall Thickness (WT), and Schedule numbers all seem to overlap. In this guide, we break down the complete steel pipe sizing system.
What Is Nominal Bore (NB/NPS)?
Nominal Bore, also called Nominal Pipe Size (NPS), describes the approximate internal diameter. For NPS 1/8″ through NPS 12″, the OD is fixed and only wall thickness changes with schedule. The metric equivalent is DN (Diameter Nominal).
OD vs ID
In the pipe industry, all sizing is based on outside diameter. Pipes are connected by their outside through fittings and threading. Always specify OD and wall thickness when ordering.
Understanding Schedule Numbers
Schedule (SCH) is a standardized wall thickness designation by ASME B36.10. Higher schedule = thicker wall = higher pressure. For a 4″ NPS pipe: SCH 40 = 6.02mm WT, SCH 80 = 8.62mm, SCH 160 = 12.70mm.
Common Steel Pipe Sizes
| NPS | DN | OD (mm) | SCH 40 WT | SCH 80 WT |
|---|---|---|---|---|
| 1/2″ | 15 | 21.3 | 2.77mm | 3.73mm |
| 1″ | 25 | 33.4 | 3.38mm | 4.55mm |
| 2″ | 50 | 60.3 | 3.91mm | 5.54mm |
| 4″ | 100 | 114.3 | 6.02mm | 8.56mm |
| 8″ | 200 | 219.1 | 8.18mm | 12.70mm |
| 12″ | 300 | 323.8 | 10.31mm | 15.09mm |
Weight Formula
W = 0.02466 x (OD – WT) x WT in kg/m (OD and WT in mm). This helps estimate shipping costs and structural loads.
Ordering Tips
Always confirm: NPS/DN size, schedule or WT, material grade, length, end type, and coating. Contact CoreMetal for sizing assistance.
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