Piping Schedules & Wall Thickness
Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes. The "Schedule" dictates the wall thickness. As the schedule number increases, the wall thickness increases, which reduces the inside diameter (ID) while the outside diameter (OD) remains constant.
| Nominal Pipe Size (NPS) | Outside Diameter (OD) | Schedule 40 (STD) Wall | Schedule 80 (XH) Wall | Schedule 160 Wall |
|---|---|---|---|---|
| 1" | 1.315" | 0.133" | 0.179" | 0.250" |
| 2" | 2.375" | 0.154" | 0.218" | 0.344" |
| 4" | 4.500" | 0.237" | 0.337" | 0.531" |
| 6" | 6.625" | 0.280" | 0.432" | 0.719" |
| 8" | 8.625" | 0.322" | 0.500" | 0.906" |
| 12" | 12.750" | 0.406" | 0.688" | 1.312" |
Why does the OD stay constant?
Keeping the Outside Diameter (OD) constant across all schedules is critical because it ensures that pipe fittings, flanges, and threading tools (dies) fit universally regardless of the pipe's internal pressure rating. If you need to handle higher pressure, you use a higher schedule, which means the pipe gets thicker inwards.