In pipeline engineering, the sizes of piping components are designated by two systems: NPS and DN. NPS is referred to as nominal pipe size which is the US customary system while DN is referred to as nominal diameter which is the used in international standards. O.D is the outside diameter of the piping component which can be actually measured. Their relationships are listed in below table.

NPS	DN	O.D
1/2	15	21.3
3/4	20	26.7
1	25	33.4
1-1/4	32	42.2
1-1/2	40	48.3
2	50	60.3
2-1/2	65	73.0
3	80	88.9
4	100	114.3
5	125	141.3
6	150	168.3
8	200	219.1
10	250	273.0
12	300	323.8
14	350	355.6

*For NPS ≥4, the related DN equals to 25 multiplied by the NPS number. The unit for O.D is mm.
*For NPS ≥14, the actual outside diameter O.D equals to 25.4 multiplied by the NPS number.