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	2.374	60.3	2.375
2 1/2	76.1	2.996	73.0	2.875
3	88.9	3.500	88.9	3.500
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