Nominal Pipe Sizes & Schedules (ASME B36.10M)
Direct Answer
Nominal Pipe Size (NPS) and Schedule designations under ASME B36.10M standardize the outer diameter, wall thickness, and pressure capabilities of forged steel, wrought iron, and stainless pipe. For NPS 1/8 to NPS 12, the nominal number represents approximate inside diameter, while maintaining a standardized fixed outer diameter across Schedule 40 (Standard) and Schedule 80 (Extra Strong) walls.
Source: ASME B36.10M-2018 (Welded and Seamless Wrought Steel Pipe)
Specs
| Governing Standard | ASME B36.10M / ASTM A53 / ASTM A106 |
| Metric Equivalent | DN (Diamètre Nominal) in millimeters (e.g., NPS 1/2 = DN 15) |
| NPS 1/2" Dimensions | OD: 0.840" (21.3 mm) | Sch 40 Wall: 0.109" (2.77 mm) | Sch 80 Wall: 0.147" (3.73 mm) |
| NPS 3/4" Dimensions | OD: 1.050" (26.7 mm) | Sch 40 Wall: 0.113" (2.87 mm) | Sch 80 Wall: 0.154" (3.91 mm) |
| NPS 1" Dimensions | OD: 1.315" (33.4 mm) | Sch 40 Wall: 0.133" (3.38 mm) | Sch 80 Wall: 0.179" (4.55 mm) |
| NPS 2" Dimensions | OD: 2.375" (60.3 mm) | Sch 40 Wall: 0.154" (3.91 mm) | Sch 80 Wall: 0.218" (5.54 mm) |
| Schedule Transition Boundary | For NPS ≥ 14, Nominal Pipe Size equals the exact Outer Diameter in inches |
The Fixed Outer Diameter Principle
In the ASME pipe sizing system, outer diameter (OD) is fixed for any given NPS designation, regardless of the schedule (wall thickness). Increasing the schedule from Schedule 40 (Standard Weight / STD) to Schedule 80 (Extra Strong / XS) increases the pipe wall inward, reducing the internal cross-sectional flow area while dramatically raising working pressure capacity. This ensures standard threading dies, roller groovers, and pipe flanges mate with any pipe schedule of the same nominal size.
NPS to DN (Metric) Conversion Matrix
International metric installations designate pipe sizes using DN (Diamètre Nominal) in millimeters:
- $NPS\; 1/2\text{"}$ $\rightarrow$ $DN\; 15$ ($OD\; 21.3\text{ mm}$)
- $NPS\; 3/4\text{"}$ $\rightarrow$ $DN\; 20$ ($OD\; 26.7\text{ mm}$)
- $NPS\; 1\text{"}$ $\rightarrow$ $DN\; 25$ ($OD\; 33.4\text{ mm}$)
- $NPS\; 1\text{-}1/4\text{"}$ $\rightarrow$ $DN\; 32$ ($OD\; 42.2\text{ mm}$)
- $NPS\; 1\text{-}1/2\text{"}$ $\rightarrow$ $DN\; 40$ ($OD\; 48.3\text{ mm}$)
- $NPS\; 2\text{"}$ $\rightarrow$ $DN\; 50$ ($OD\; 60.3\text{ mm}$)
- $NPS\; 3\text{"}$ $\rightarrow$ $DN\; 80$ ($OD\; 88.9\text{ mm}$)
- $NPS\; 4\text{"}$ $\rightarrow$ $DN\; 100$ ($OD\; 114.3\text{ mm}$)