Pipe or tubing... which is it?  Does it matter, and if so, why?

Pipe is a kind of tubing, so what exactly is the difference?  The difference is subtle, yet important whenever you are specifying or discussing the use of one or the other....

• Pipe is designated by Trade Size, aka nominal diameter (roughly according to its Inside Diameter). Pipe is not normally Flexible.
• Tubing is designated by actual Outside Diameter. Tubing may be either Flexible or Rigid.
• Hose is designated by its actual Inside Diameter. Hose is not normally Rigid.

Pipe is designated by trade size, and generally has an inside diameter somewhere near its nominal size.  

For instance, a standard 3/4-inch schedule 80 iron pipe (whether its made of iron, steel, brass,etc.) has an O.D. of about 1.05 inches (nowhere near 3/4 inch), and an I.D. of .742 inches - very nearly 3/4 inch.  On the other hand, 3/4 hose has, by definition, a .750 inside diameter, and an outside diameter depending only on its composition and pressure handling capacity.  (See our book page, American National Pipe for more details).

To handle more pressure, pipe "grows" inwardly, whereas hose grows outwardly.  This is a result of their respective fitment constraints.  A given size of pipe must always be able to thread into its mating appliance. Therefore its O.D. must remain constant as its wall thickness is varied. A given size of hose, on the other hand, must always be able to stretch over its mating appliance (typically a Hose Barb).

Tubing, on the other hand is fitted differently, so it is designated by its actual outside diameter.  For instance, a 3/8 JIC hydraulic flare fitting will have a B-nut and collar that slide over the tubing prior to the forming of the flare.  This B-nut and Collar are sized to match exactly the O.D. of the tubing, so the tubing must "grow" internally to accommodate more pressure.  Tubing of this sort is designated by its actual outside diameter, so 3/8 tubing will have an OD of .375 inches, whereas 3/8 pipe will have an actual OD of about .675 inch.

Note that in the manufacturing process, a properly formed tapered thread can be cut on a pipe which may vary considerably in its actual OD, whereas a B-nut and Sleeve require a much closer tolerance fit (the same goes for a compression ferrule).  Therefore, the manufacturing tolerance of the OD is much tighter for tubing than for pipe.

• A Note Regarding Hose...

"Wait a minute," you say, "Hose is designated by inside diameter."  That's correct, but technically hose is pipe, not tubing.  In fact, in Europe and the U.K. it is commonly referred to as "hose pipe."  The subtle difference between hose and pipe is that, whereas pipe (e.g., iron pipe) is sized by exact constraints on its outside diameter, hose is sized by exact (hopefully) constraints on its inside diameter.  This is a result of the way it is typically fitted.  In other words, the fitments of pipe are made via threads (usually tapered) cut onto the outside diameter of the pipe, whereas fitments of hose are made usually via a "hose barb," wherein the hose is mildly stretched over an internal hard nipple on the adjoining appliance (usually a fitting).