Pipe schedule chart
Read across: outside diameter is constant, and the schedule column gives the wall. Internal diameter is the outside diameter minus twice the wall.
This is the whole point of the schedule system. Because the outside diameter never moves, the same threading dies, fittings and flanges fit any schedule of a given size.
| NPS | OD | Sch 10 wall | Sch 40 wall | Sch 80 wall | Sch 40 ID |
|---|---|---|---|---|---|
| 1/2 | 0.840 | 0.083 | 0.109 | 0.147 | 0.622 |
| 3/4 | 1.050 | 0.083 | 0.113 | 0.154 | 0.824 |
| 1 | 1.315 | 0.109 | 0.133 | 0.179 | 1.049 |
| 1 1/4 | 1.660 | 0.109 | 0.140 | 0.191 | 1.380 |
| 1 1/2 | 1.900 | 0.109 | 0.145 | 0.200 | 1.610 |
| 2 | 2.375 | 0.109 | 0.154 | 0.218 | 2.067 |
| 2 1/2 | 2.875 | 0.120 | 0.203 | 0.276 | 2.469 |
| 3 | 3.500 | 0.120 | 0.216 | 0.300 | 3.068 |
| 4 | 4.500 | 0.120 | 0.237 | 0.337 | 4.026 |
| 6 | 6.625 | 0.134 | 0.280 | 0.432 | 6.065 |
| 8 | 8.625 | 0.148 | 0.322 | 0.500 | 7.981 |
| 10 | 10.750 | 0.165 | 0.365 | 0.594 | 10.020 |
| 12 | 12.750 | 0.180 | 0.406 | 0.688 | 12.000 |
Why nominal size is not a measurement
Below 12 inch, nominal pipe size is a label with no direct relationship to any dimension. A 2 inch pipe measures 2.375 inch outside. The number originally described the approximate bore of the wrought iron pipe of that era, and once fittings were standardised around it the outside diameter had to be frozen even as wall thicknesses changed.
From 14 inch upward the convention changes and nominal size equals the actual outside diameter. A 16 inch pipe really is 16.000 inch outside.
The metric equivalent is DN, which is a rounded conversion rather than a measurement either. DN50 is the 2 inch size, DN80 is 3 inch, DN100 is 4 inch.
Choosing a schedule
Schedule 40 is the general-purpose wall for water, compressed air and low-pressure process work, and it is what unqualified references to "steel pipe" usually mean.
Schedule 80 roughly doubles the pressure rating and is used where pressure, mechanical abuse or thread depth demands it. Threading eats into the wall, so a threaded joint in schedule 40 has noticeably less metal left than the plain pipe suggests.
Schedule 10 is thin-wall for low-pressure and roll-groove work, common in stainless process and sprinkler systems. It cannot be threaded.
