What Is OD and ID?

OD (Outside Diameter) is the external measurement across the pipe cross-section. ID (Inside Diameter) is the internal bore through which fluid flows. For any given NPS, the OD is fixed and standardized per ASME B36.10 or B36.19. The ID changes with wall thickness (schedule).

The relationship: ID = OD - 2 x WT

This means an NPS 6 pipe always has an OD of 168.3 mm regardless of schedule. A Schedule 40 NPS 6 pipe has a wall thickness of 7.11 mm, giving an ID of 154.1 mm. A Schedule 160 NPS 6 pipe has a wall thickness of 18.26 mm, giving an ID of only 131.8 mm.

NPS	DN	OD (mm)	SCH 40 WT (mm)	SCH 40 ID (mm)	SCH 80 WT (mm)	SCH 80 ID (mm)
1/2	15	21.3	2.77	15.8	3.73	13.8
1	25	33.4	3.38	26.6	4.55	24.3
2	50	60.3	3.91	52.5	5.54	49.2
3	80	88.9	5.49	77.9	7.62	73.7
4	100	114.3	6.02	102.3	8.56	97.2
6	150	168.3	7.11	154.1	10.97	146.4
8	200	219.1	8.18	202.7	12.70	193.7
10	250	273.1	9.27	254.6	15.09	242.9
12	300	323.9	10.31	303.3	17.48	288.9
16	400	406.4	12.70	381.0	21.44	363.5
24	600	610.0	17.48	575.0	30.96	548.1

Why OD Is Fixed

The OD must be constant for a given NPS so that all connecting components, including flanges, buttweld fittings, and branch connections, fit together regardless of pipe schedule. A flange bore matches the pipe OD. Changing the OD would break the interchangeability of the entire piping system.

Wall thickness (and therefore ID) varies to meet different pressure and temperature requirements.

ID and Flow Area

The ID directly determines the flow cross-sectional area and therefore the fluid velocity for a given flow rate. Increasing the wall thickness reduces the ID and increases velocity.

NPS 6 Schedule	ID (mm)	Flow Area (cm2)	Velocity at 100 m3/h
SCH 10	162.7	208.0	1.34 m/s
SCH 40 (STD)	154.1	186.5	1.49 m/s
SCH 80 (XS)	146.4	168.3	1.65 m/s
SCH 160	131.8	136.5	2.04 m/s

Pro Tip

Process engineers size pipes based on allowable velocity and pressure drop, which depend on the ID. Piping engineers then select the schedule that provides enough wall thickness for the design pressure and temperature. Always verify that the selected schedule does not reduce the ID below the hydraulic design basis. This check is especially critical for heavy-wall schedules (SCH 120+) in smaller pipe sizes.

OD for Tubes vs. Pipes

Tubes are specified by actual OD and wall thickness, not by NPS. A 1-inch tube has an actual OD of 25.4 mm. An NPS 1 pipe has an OD of 33.4 mm. They are not interchangeable.

Item	Size Designation	Actual OD
NPS 1 pipe	NPS 1 / DN 25	33.4 mm
1” tube	1” OD	25.4 mm
NPS 2 pipe	NPS 2 / DN 50	60.3 mm
2” tube	2” OD	50.8 mm

This distinction is critical when connecting instrumentation tubing to process piping. Compression fittings are sized for actual tube OD, while pipe fittings are sized for NPS OD.

Read the full guide to pipe types