Pipe vs Tube FAQ: Understanding the Differences

Wait! There is a difference between Pipe and Tube?

Knowing the answer to this question and how to properly measure each will win you trivia championships!
 
Tube and Pipe are not the same. In simple terms, pipe is a vessel used to most often carry fluids and gas. Tube is used for structural projects like car frames and construction. Once you know what you want the next step is figuring out the size you need.
 
Tube is measured by an exact outside diameter (OD) and wall thickness.
Pipe is measured by a nominal outside diameter (also known as NPS or Nominal Pipe Size) and wall thickness.

NPS is a North American set of standard sizes for pipes used for high or low pressures. The schedule number on a pipe relates to the thickness of the wall on the pipe. The higher the number, the thicker the wall. So Schedule 80 pipe is thicker walled material than Schedule 40 pipe. The Schedule number can be the same on two different sized pipes and yet the actual wall thickness will be different. For example a 1” Schedule 40 pipe has a wall thickness of .133”. Whereas a 2” Schedule 40 pipe has a wall thickness of .145”. So, the bottom line on pipe is that the Schedule number will increase the wall thickness, and the wall thickness changes based on the NPS
 
The dimensions provided for tubing refer to the actual outside diameter. In other words, the actual physical OD of a tube is just the same as it's nominal OD. The size of a tube will keep the same OD no matter what the wall thickness is. For example, the actual outside diameter of 1¼″ pipe is 1.625″ - while 1¼″ tube has a true 1.25″ outside diameter.

Consequently, both the size of tube and pipe is measured by it's OD and the thickness.

You have probably heard the terms pipe and tube used interchangeably, and there's a good chance you aren't exactly sure what the difference is between them. Most people, even experts working in related industries, don't actually know. After all, how different can hollow cylinders get, and who really cares enough to go and look? Well, I had to know...

Even looking on the internet can lead to confusion, as many websites have mistakenly misidentified the real differences between them. Well, since I'm a know-it-all, let me explain as best I can what exactly differentiates tube and pipe. There's a nifty looking infographic you can share at the bottom.

Tubing

Tubing is generally used for structural purposes and the OD is an important and exact number. Tubing size is specified by OD and WT (wall thickness). The stated OD and measured OD are generally within very close tolerances of each other. Tubing is usually more expensive than pipe due to tighter manufacturing tolerances.

Interestingly, while the stated and measured OD’s of tubing are almost exact in most cases, copper tubing generally has a measured OD that is 1/8” larger than stated OD. As such, maybe it should be called copper pipe. However, stainless steel, aluminum, and steel tubing all have measured and stated OD’s that are exact or within a specified tolerance, usually in the thousandths of an inch (e.g. +/- 0.04").

Pipe

Pipes are categorized as tubular vessels used in pipeline and piping systems, and commonly transport gases and fluids. They are specified by “Nominal Pipe Size” (NPS) and Schedule (wall thickness). NPS is a size standard established by the American National Standards Institute (ANSI), and should NOT be confused with the various thread standards such as NPT and NPSC.

The manufacturing of Nominal Pipe Sizes from 1/8” to 12” is based on a standardized nominal outside diameter (OD) that is different from the measured OD. NPS pipe 14” and up have measured OD’s that correspond to the nominal size.

The reason many people, including plumbers, engineers, and others, mistakenly believe NPS refers to the ID on smaller pipes is because of how the standard was originally defined. The standardized OD was originally defined so that a pipe with a standardized OD and wall thickness typical of the period when it was established, would have a pipe ID that was approximately equal to the nominal size of the pipe. For example, 3” Schedule 40 NPS has an outside diameter and wall thickness that very roughly gives it an inside diameter of 3”. Regardless of the wall thickness the nominal OD of a pipe will not change.

The Takeaway

The long and short is that the NPS was defined so you would know approximately what ID you were working with and could count on the OD never changing. Tubing is defined so you know exactly what OD you are working with and the ID is based on whatever the wall thickness is.

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