Be careful when specifying Nominal Pipe Sizes. They could either be USA sizes, or BSP (British Standard Pipe) sizes which are similar to but NOT the same. Generally, when a thread specification is called out on a pipe, it will determine which system is needed. However, this is not an absolute. Many of the pipe sizes will support either the BSP or USA (NPT) threads.
**Depending upon the standard number, this may also be 168.3 mm
Nominal Pipe Size |
BSP pipe O.D. in mm |
BSP pipe O.D. in inches |
USA pipe O.D. in mm |
USA pipe O.D. in inches |
1/8 | 10.2 | 0.402 | 10.3 | 0.405 |
1/4 | 13.5 | 0.531 | 13.7 | 0.540 |
3/8 | 17.2 | 0.677 | 17.1 | 0.675 |
1/2 | 21.3 | 0.839 | 21.3 | 0.840 |
3/4 | 26.9 | 1.059 | 26.7 | 1.050 |
1 | 33.7 | 1.327 | 33.4 | 1.315 |
1 1/4 | 42.4 | 1.669 | 42.2 | 1.660 |
1 1/2 | 48.3 | 1.902 | 48.3 | 1.900 |
2 | 60.3 | 2.375 | 60.3 | 2.375 |
2 1/2 | 76.1 | 2.996 | 73 | 2.875 |
3 | 88.9 | 3.500 | 88.9 | 3.500 |
3 1/2 | 101.6 | 4.000 | 101.6 | 4.000 |
4 | 114.3 | 4.500 | 114.3 | 4.500 |
5 | 139.7 | 5.500 | 141.3 | 5.563 |
6 | 165.1** | 6.500 | 168.3 | 6.625 |
8 | 219.1 | 8.626 | 219.1 | 8.625 |
10 | 273 | 10.75 | 273 | 10.75 |
12 | 323.9 | 12.75 | 323.8 | 12.75 |
14 | 355.6 | 14.000 | 355.6 | 14.000 |
16 | 406.4 | 16.000 | 406.4 | 16.000 |
18 | 457 | 17.992 | 457.2 | 18.000 |
20 | 508 | 20.000 | 508 | 20.000 |