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