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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 |
**Depending upon the standard number, this may also be 168.3 mm
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