The Difference in the Details: OD vs ID vs Nominal Pipe Size

If trying to determine the actual dimensions of a pipe inside diameter or pipe outside diameter have you running for a pipe size chart, you've run into one of the common issues with measuring the diameter of a pipe.

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Given the many differences between pipes and tubes, schedule numbers such as schedule 40 or schedule 80, and different materials including copper tubing and stainless steel pipes, determining the pipe OD and pipe ID can be difficult at best. In this article, we'll explore the differences between each and how to determine the diameter of the pipe you're working on to make the process easier.

Pipes versus Tubing

Let's start by defining the difference between piping and tubing:

A pipe is round and is measured by the nominal pipe size, along with a schedule which designates the wall thickness.

To create a piping system designed to transport fluids or gases, pipes are connected to pipe nipples, flanges, fittings, and valves.

Tubing is round, but can also be square or rectangular, as well as measured by wall thickness and outside diameter (OD). The outside diameter is exact meaning a 1.25" tube is exactly 1.25" at the OD. The wall thickness is the actual thickness or is designated by a gauge. Tubing can be round, rectangular, or square.

What does outside diameter mean?

The outer diameter (also known as external diameter) of a pipe is the measurement from one outside edge to the opposite outside edge, which measures the maximum width of the outside of the pipe.

This measurement is especially important in close spaces, as it may limit the amount of piping you can place in locations with limited available space.

The nominal diameter defines the size of the pipe; but it is dimensionless. For example, the nominal size of 4” schedule 80 pipe is 4”; but the outside diameter is actually 4.5”. It is important to note that the outside diameter is not dependent on its thickness.

What is inside diameter?

Inside diameter measures the distance from one inside edge of a pipe to the opposite inside edge, measuring the maximum measurement of the inside of the pipe. This measurement is important as it will impact how much material the pipe can contain at any one time, which can impact flow rates.

As an example, a 1" pipe will have the same outside diameter of 1.315", but a schedule 40 pipe will have a wall thickness of 0.133" while a schedule 80 pipe of the same outside diameter will have a wall thickness of 0.179", restricting internal flow.

What is nominal pipe size?

Often incorrectly called national pipe size or confused with national pipe thread, nominal pipe size (NPS) is a dimensionless number identifying the "size" of the pipe; generally found in steel pipe, PVC, iron pipe, and similar materials.

For example, an NPS 2” pipe has an OD of 2.375” while an NPS 6” pipe has an OD of 6.625”, and an NPS 12” pipe has an OD of 12.75”. However, NPS 14” and above has an OD that matches their NPS designation. This means an NPS 14” pipe has an OD of 14”, an NPS 16” pipe has an OD of 16” and so on.

Chart in Inches (In.)

Nominal Pipe Size	Outside/Outer Diameter (OD)	Schedule 10		Schedule 40		Schedule 80		Schedule 120		Schedule 160
Nominal Pipe Size	Outside/Outer Diameter (OD)	Inner/Inside Diameter (ID)	Wall Thickness	Inner/Inside Diameter (ID)	Wall Thickness	Inner/Inside Diameter (ID)	Wall Thickness	Inner/Inside Diameter (ID)	Wall Thickness	Inner/Inside Diameter (ID)	Wall Thickness
1/8"	0.405	0.307	0.049	0.269	0.068	0.215	0.095	--	--	--	--
1/4"	0.540	0.410	0.065	0.364	0.088	0.302	0.119	--	--	--	--
3/8"	0.675	0.545	0.065	0.493	0.091	0.423	0.126	--	--	--	--
1/2"	0.840	0.674	0.083	0.622	0.109	0.546	0.147	0.480	0.170	0.464	0.188
3/4"	1.050	0.884	0.083	0.824	0.113	0.742	0.154	0.690	0.170	0.612	0.219
1"	1.315	1.097	0.109	1.049	0.133	0.957	0.179	0.891	0.200	0.815	0.250
1-1/4"	1.660	1.442	0.109	1.380	0.140	1.278	0.191	1.204	0.215	1.160	0.250
1-1/2"	1.900	1.682	0.109	1.610	0.145	1.500	0.200	1.423	0.225	1.338	0.281
2"	2.375	2.157	0.109	2.067	0.154	1.939	0.218	1.845	0.250	1.687	0.344
2-1/2"	2.875	2.635	0.120	2.469	0.203	2.325	0.275	2.239	0.300	2.125	0.375
3"	3.500	3.260	0.120	3.068	0.216	2.900	0.300	2.758	0.350	2.624	0.438
3-1/2"	4.000	3.760	0.120	3.548	0.226	3.364	0.318	--	--	--	--
4"	4.500	4.260	0.120	4.026	0.237	3.826	0.337	3.574	0.473	3.438	0.531
5"	5.563	5.295	0.134	5.047	0.258	4.813	0.375	--	--	4.313	0.625
6"	6.625	6.357	0.134	6.065	0.280	5.761	0.432	--	--	5.187	0.719
8"	8.625	8.329	0.148	7.981	0.322	7.625	0.500	--	--	6.813	0.906

Please refer to the above table for actual pipe dimensions. The outer diameter is different than the nominal pipe size. Based on the pipe's nominal size , the above pipe schedules refers to the wall thickness .

What does a pipe schedule mean?

A pipe schedule is another dimensionless number that refers specifically to the wall thickness of a pipe, which in turn designates its ability to withstand pressure. In general, the higher the schedule number, the thicker the pipe wall, and the better able it is to withstand pressure from air, gas, or liquids.

For this reason, a schedule 40 pipe will be able to withstand less pressure than a schedule 80 pipe but may be able to carry more material at the same time and pressure due to a larger inner diameter.

While there are multiple pipe schedules (5, 10, 40, 80, and others), the wall thickness is not uniform across a given schedule. For schedule 40 an NPS 2” pipe will have a wall thickness of 0.154”, while an NPS 6” pipe will have a wall thickness of 0.280”. These same NPS sizes will have thicker walls if the pipe was schedule 80; NPS 2” would be 0.218” and NPS 6” would be 0.432”.

Nominal Pipe Size vs OD vs ID

What about NB nominal bore?

Nominal bore is another term for nominal pipe size which is used as the piping standard in the European Union as their designation equivalent to NPS. Nominal bore measures the interior diameter or ID of the pipe.

It may also appear as diamètre nominal, or DN, referring to the pipe ID in millimeters.

By having a better understanding of how the diameter ID, OD, and NP correlate to each other, you can better determine how much space the pipes will take up, how much material they can carry at a time, and what kind of pressure they can withstand.

If you have questions about your piping and need help finding an appropriate match for your application, the experienced professionals at Merit Brass are ready to help.

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AUTHOR: Marcus Estrella, Merit Brass - Director of Product Line Management and Kimberly Wallingford, Merit Brass - Director of Marketing