BSP/USA pipe O.D. (outside diameter) comparison chart
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.|
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 |