How to Understand Pipe Sizes

[A version of this article was originally published in the NOMMA Fabricator]

Is it pipe or is it tube? Understanding the differences can make sure you get the material you want in the size you need.

Do you remember being surprised when you learned that a “2 by 4″ doesn’t actually measure 2 inches by 4 inches? Have you ever been told that 1-1/8” pipe doesn’t exist? Using the correct terminology when ordering material (or fittings, tools, or other items that must be used with these materials) can save a lot of time, headaches and money!

Many products have a name that for convenience only approximates the material’s size. These are sometimes referred to as nominal dimensions. Webster’s describes nominal as “in name only.” In other words, you can’t trust the “name” dimensions in actual measurements or calculations. Differences and difficulties in correctly describing a tube and pipe are common in the metalworking industry.

Pipe is a commonly used material in the fencing industry, most commonly for chain link fences and gates. However pipe and tubing are not the same materials! Pipe was originally used for the movement of water, and therefore the ID (inside diameter) was the critical dimension. The nominal dimension for pipe is the ID. So, 1½ inch pipe is NOT 1½ inch outside diameter, but instead is nominally, (approximately) 1½ inch inside diameter. Pipe is typically manufactured to looser tolerances and less expensive to purchase. However in the fence and construction industry, where pipe is commonly used, it will often be referred to by its outside dimension, since fittings, caps and other parts most commonly mate the the outside of the pipe.

The wall thickness of pipe is designated by various “schedules,” most commonly schedule 5, 10, and 40. The exact wall thickness of anyone schedule changes with the pipe size. A 1 inch Sched. 40 pipe has a .133 inch wall, but a 2 inch Sched. 40 pipe has a wall thickness of .154 inches.

Tubing, on the other hand, is typically produced to tighter tolerances and designed for consistent mechanical and structural properties. Because it is typically more costly to produce it is seldom used for chain link fencing, scaffolding, etc.  To further complicate matters, some companies market pipe to the fence industry as “fence tubing.” The thickness of a tube’s wall is normally described as a gauge. While a specific pipe schedule will mean different wall thicknesses depending on the pipe size, a specific gauge is consistent regardless of tube OD (outside diameter).

Let’s look at a typical example…

A purchasing agent is told to buy a “2 inch pipe notcher,” there are actually many different sizes that could possibly fit this description, as shown below.