DT logo holding tube and pipe

Wait…. What is The Difference Between Tube And Pipe? It’s probably a question you never asked until just now. For most the terms are interchangeable or they have a simple (but wrong) rule of thumb like “pipe is for fluid, tube is for structure”. In reality the answer is simple, but also a very interesting journey on why we got to having two different systems in use at the same time for cylindrical shaped hollow objects.

Let’s explore the baffling differences to make it incredibly easy to understand.

BLUF The Definitions:

Tube - The logical standard

Tube is the straightforward, no-nonsense definition. Its specifications are defined by actual, literal dimensions:

Tube is defined by its Outside Dimension (OD)

Wall Thickness (W).

Inside diameter is inferred by subtracting the wall thickness x2.

For example, a 1-inch tube with a 0.050-inch wall will have an OD that is exactly 1.0 inch. The Inside Dimension (ID) is a simple calculation: 1.0 inch minus two times the wall thickness (0.90 inches). Tube is precise, accurate, and literal. There is no official standards body for tube; what is on the label is what is delivered. This freedom and precision allow engineers, especially in weight-critical industries like aerospace, to design components to exacting, tight-margin specifications. Any modern material like carbon fiber or flexible tubing will almost always be measured in tube dimensions

Pipe - The Historic Mash-up

Pipe, on the other hand, operates on the confusing principle of Nominal Pipe Size (NPS)—a standard that is literally dimensionless. It means the number on the label doesn’t actually measure the pipe’s size. Pipe states that for a given nominal size (e.g., “1-inch pipe”), the Outer Diameter (OD) stays the same(but is not listed), and a “schedule” (like Schedule 40 or 80) dictates the wall thickness. The Inside Diameter (ID) is only a rough approximation of the nominal size.

For instance, a 1-inch Schedule 80 pipe is manufactured with a 1.315-inch OD, a 0.179-inch wall thickness, and a 0.957-inch ID. It’s Iclose* to 1 inch, but not exact. The term “Nominal” itself means “in name only” which is essentially engineering speak for saying “don’t take this literally”.

To determine the exact OD or ID for any level of precision, one must consult a reference chart because the number on the label cannot be trusted. So that begs the question why use a dimensionless standard and how did we get there

Pipe Standards A Tale from the 1800’s

The reason for this dual system is a fascinating and wild piece of industrial history.

In the 1800s, steel started becoming a prominent building material. Starting with cast and wrought iron in the early 1800’s and moving to Steel with the invention of the Bessemer process in 1856. Before modern transportation steel mills would be mostly regional and standardization wasn’t in full force. That means a set of pipe made in Chicago may not match pipe made in Philadelphia or New York.

During the Industrial Revolution, water and steam pipes needed to be standardized for mass production. Since the pipes were primarily for carrying fluid, the most critical dimension was the Inside Dimension (ID), as this determined the flow rate. Also before welding pipe was generally joined by threaded connections which mean you would oversize the OD so that you could machine in threads and get close to the nominal ID.. This led to the creation of the Iron Pipe Size (IPS) system.

The push for standardization came to a head around the American Civil War in 1862. An engineer named Robert Briggs observed that steel mills in Philadelphia were using different dies for the same size pipe, making them non-interchangeable. Briggs mandated a standard based on the actual dies being used at the Pascal Ironworks where he worked. This became the Briggs Standard which was in wide use by 1887. Which merged with IPS, and is why pipe dimensions today do not literally measure what the label says—they are based on the historical dimensions of a set of dies used to make threaded pipe in the 1860’s at one steel mill in Philadelphia!

Later, in 1939, as industrial needs grew. The original IPS system was did not have enough pipe schedules, it only had Standard, Extra Strength (XS) and wait for it… Extra Extra Strength (XXS). As machines got more complex and with higher performance the American Standards Association (now ANSI) developed Nominal Pipe Size (NPS). This system retained the original ODs from the Briggs and IPS standards but filled in many of the blanks and created more schedules. It also introduced a formula to define a wall thickness (pipe schedule). It’s a simple ratio of the service pressure divided by the allow stress in the material, then multiplied by 1000 so the numbers weren’t small. Just like IPS the OD remained constant for a given nominal size, ensuring that flanges and fittings would still connect properly, regardless of the wall schedule.

But Here’s Where the System Breaks Down…

The combination of a nominal system and 1800s holdovers results in a confusing set of standards that started accumulating more and more caveats.

The Switch to Tube: As pipe sizes got larger the nominal dimensions started getting further off. So below 12 inches, pipe dimensions are nominal. However, for 14-inch pipe and above, the measurement switches to tube dimensions. The Nominal size becomes the true OD. 12 Inch pipe is 12.75 inch OD while 14 inch pipe is 14.0 inch OD

Multiple different pipe schedules can have the same dimensions: Because the wall thickness changes the ID, multiple different sizes can have the same ID. For example 16 inch Schedule 30, 40S, and Standard (STD) all have 0.375 wall thickness and result in an actual ID of 15.25 inches. However 18 inch schedule 120 also has a 15.25 inch ID. This can easily create confusion. Also you now have 3 different schedules (30,40S, and Standard) that all have the EXACT same measurements, but at different pipe sizes these change.

The Stainless Steel Problem: common Stainless steel grades are generally stronger than carbon steel used for pipe. Since the wall schedule is a ratio of based on the strength of the material, It’s wall thicknesses should be thinner. To ease manufacturing, schedules for stainless and carbon steel pipes 12 inches and below are the same. But above 12 inches, they are different. This means two separate products can both be labeled “14-inch Schedule 40” yet have different IDs and wall thicknesses. To alleviate the confusion the add an “S” to the chart to mention that it’s for stainless steel, but in common speaking people will say “8 inch schedule 40 stainless steel” and won’t remember to say the S.

Don’t worry it gets crazier when we move to copper.

The Copper Conspiracy: Copper pipe uses a system called Copper Tube Size (CTS), which oddly measures what is commonly called copper pipe. It follows a ⅛th-inch rule, where the OD is ⅛th of an inch larger than the nominal ID (e.g., a 0.5-inch pipe is ⅝ths inches OD).

To make things more complicated, the ACR (Air conditioning and Refrigeration) Tube system uses actual tube dimensions. So ½-inch schedule L copper pipe, is dimensionally equivalent to a ⅝-inch ACR Tube! the same piece of metal is called by two different names, measured two different ways, based on its intended use. The confusion between pipe and tube standards happens ALL THE TIME It’s very easy to hear “1 inch pipe” and then purchase 1 inch tube without realizing the dimensions will be different.

The Takeaway

Though common, using any rules of thumb always break down. “pipe is for fluid and tube is for structure” this is fundamentally untrue, and since there are so many caveats in pipe dimensions I’ve never found one that doesn’t just add confusion. The difference boils down to dimensioning. Pipe is a legacy system based on historical standards and a “nominal” approximation. It’s like using pre-sized, non-custom Lego bricks. Tube is a precise, modern system that adheres to actual, literal dimensions. This difference is why mistakes are so common and so expensive. When fitting, welding, or purchasing any round metal component, it is essential to check the context, read the technical drawings, and ask for clarification.

To remember the difference, think of this: Tube is true, and pipe… just ain’t right (dimensionally).

References

1. ASME B36.10 and B36.19 The code that defines pipe sizes
2. ISO 6708: Defines pipe sizes in metric
3. Explanation of pipe schedule formula
4. Good video breaking it down
5. Copper pipe is really measured by the copper tube size system
6. Forum on copper talking about confusion between plumbing and HVAC copper https://www.hvac-talk.com/threads/refrigerant-copper-vs-plumbing-copper.2178841/
7. Useful reference of pipe schedules
8. Complete Pipe schedule chart
9. Another pipe schedule chart
10. How NPS got it’s name
11. Robert Briggs obituary
12. Copper Tube Handbook
13. Pascal Iron works picture
14. Historic pipe picture
15. US cast iron pipe foundry
16. historic pipe lithographs
17. Briggs standard dimensions
18. NPS wiki
19. Pipe thread standards
20. DN to NPS pipe sizes
21. Wrong pipe kills