NPS versus IPS - Definitions and Size Charts

NPS - Nominal Pipe Size is a North American set of standard sizes for pipe usage.

Diameter Nominal DN (mm)	Nominal Pipe Size NPS (inches)	Outside diameter (OD)  inches (mm)
6	⅛	0.405 in (10.29 mm)
8	¼	0.540 in (13.72 mm)
10	⅜	0.675 in (17.15 mm)
15	½	0.840 in (21.34 mm)
20	¾	1.050 in (26.67 mm)
25	1	1.315 in (33.40 mm)
32	1¼	1.660 in (42.16 mm)
40	1½	1.900 in (48.26 mm)
50	2	2.375 in (60.33 mm)
65	2½	2.875 in (73.02 mm)
80	3	3.500 in (88.90 mm)
100	4	4.500 in (114.30 mm)

Reference: https://www.massflow-online.com/faqs/where-do-nps-or-dn-stand-for/

IPS - Iron Pipe Size is determined by the inside diameter of the pipe.

Reference: https://www.hollaender.com/site/assets/files/4477/hollaender_pipe_size_chart_2017.pdf 

3/4" IPS
1.050", 26.67mm O.D.
Schedule 40: 0.824" I.D., 0.113" Wall

1" IPS

1.315", 33.401mm O.D.
Schedule 40: 1.049" I.D., 0.133" Wall

1 - 1/2” IPS

1.900”, 48.26mm O.D.

Schedule 40: 1.610” I.D., 0.145” Wall
Schedule 80: 1.500” I.D., 0.200” Wall

1 - 1/4" IPS

1.660", 42.164mm O.D.
Schedule 40: 1.380" I.D., 0.140" Wall

2” IPS

2.375”, 60.325mm O.D.
Schedule 40: 2.067” I.D., 0.154” Wall
Schedule 80: 1.939” I.D., 0.218” Wall