For a given Nominal Pipe Size (NPS), the Outside Diameter (OD) is fixed. The Schedule number determines the wall thickness. A higher schedule (like Sch 80 vs Sch 40) means a thicker wall, which inherently reduces the Inside Diameter (ID). Schedule is a dimensional standard, not a direct pressure rating.
| Concept | Outside Diameter (OD) | Wall Thickness (t) | Inside Diameter (ID) |
|---|---|---|---|
| NPS (Nominal) | Fixed for a specific NPS | Varies by Schedule | Decreases as Schedule increases |
| Schedule 40 | Standard (STD) | Standard Wall | Standard Flow Area |
| Schedule 80 | Standard (STD) | Extra Heavy (XS) | Reduced Flow Area |
Commonly used for instrument air, domestic water lines, cooling loops, and auxiliary process lines. These sizes are typically joined using threaded or socket-weld fittings rather than butt-welding.
| Schedule 40 | Schedule 80 | ||||
|---|---|---|---|---|---|
| NPS | OD (mm) | Sch 40 Wall | Sch 40 ID | Sch 80 Wall | Sch 80 ID |
| 1/2" | 21.3 | 2.77 | 15.76 | 3.73 | 13.84 |
| 3/4" | 26.7 | 2.87 | 20.96 | 3.91 | 18.88 |
| 1" | 33.4 | 3.38 | 26.64 | 4.55 | 24.30 |
| 1-1/4" | 42.2 | 3.56 | 35.08 | 4.85 | 32.50 |
| 1-1/2" | 48.3 | 3.68 | 40.94 | 5.08 | 38.14 |
| 2" | 60.3 | 3.91 | 52.48 | 5.54 | 49.22 |
The workhorse of industrial process piping. Used for main product lines, cooling water systems, and steam distribution. Usually joined using butt-welded fittings and standard ANSI flanges.
| Schedule 40 | Schedule 80 | ||||
|---|---|---|---|---|---|
| NPS | OD (mm) | Sch 40 Wall | Sch 40 ID | Sch 80 Wall | Sch 80 ID |
| 2-1/2" | 73.0 | 5.16 | 62.68 | 7.01 | 58.98 |
| 3" | 88.9 | 5.49 | 77.92 | 7.62 | 73.66 |
| 4" | 114.3 | 6.02 | 102.26 | 8.56 | 97.18 |
| 5" | 141.3 | 6.55 | 128.20 | 9.52 | 122.26 |
| 6" | 168.3 | 7.11 | 154.08 | 10.97 | 146.36 |
Used for main headers, transmission lines, and high-volume fluid transport. The weight of these pipes becomes a significant factor in structural support and pipe rack design.
| Schedule 40 | Schedule 80 | ||||
|---|---|---|---|---|---|
| NPS | OD (mm) | Sch 40 Wall | Sch 40 ID | Sch 80 Wall | Sch 80 ID |
| 8" | 219.1 | 8.18 | 202.74 | 12.70 | 193.70 |
| 10" | 273.0 | 9.27 | 254.46 | 15.09 | 242.82 |
| 12" | 323.9 | 9.53 | 304.84 | 17.48 | 288.94 |