You've probably noticed that not all fittings and pipe are the same. In fact, you might have noticed that it's almost easier to consider the pipe size to be a name rather than a description, since your measuring tape doesn't seem to help you much. Really, is it too much to ask that a 1/2" fitting actually measures 1/2" somewhere?
Apparently so, but why?
The History (the short and simple version)
Once upon a time, all pipe and connectors were made by hand, just like everything else. When industrialization started happening and things could be mass produced, our ancestors decided mass producing pipe would be a great step forward and looked for a way to standardize pipe sizes. Naturally this standardization was based on the needs, materials and manufacturing abilities at the time. Since knowing how much flow something could handle was important, the inside diameter was used as the frame of reference. The outside diameter was standardized as well, so all piping could be joined together without a lot of extra fuss. In other words, 1/2" pipe had a 1/2" inner diameter and a standard wall thickness - in the case of copper pipe, it was 1/16". Great! So what happened? In short, progress happened.
Metallurgy and manufacturing processes improved and new materials were created. Thinner pipes created with this new knowledge could withstand the same rigors as the old standard. Copper was no longer the only mass produced pipe. To keep everything still fitting together easily, they kept the outside diameter the same. But thinner pipes with the same outer diameter now meant that these new pipes were no longer 1/2" anywhere. So, what to call them to make everything still work smoothly within the industry? They used the same naming convention, just to keep things more or less consistent. The result? Pipe sizes now are more of a name rather than a descriptive term. In fact, the term "nominal" which is one way of defining a pipe size, means "approximate."
What about fittings? Remember, this was supposed to be fairly intuitive, and outer diameters of pipe were standardized. So, to keep it easy, pipe fittings were called by the size pipe they would fit. In other words, for 1/2" pipe, you would have selected 1/2" fittings of the same material.
Okay, so we've got the history, and it all makes sense, so why is it so confusing now?
Remember those new materials and technologies? And the progress? Well, not only did piping tech improve, so did the stuff it was meant to service, and so there were even more changes.
Because of these changes different measurements of pipe became standardized as well. This may have been a way to conserve materials since low or no pressure systems obviously don't need the heavy duty reinforcing that pressurized systems would need. It also could have been the plumbing equivalent of the media format wars (do you remember Beta vs VHS?). However it happened, more categories of piping evolved, along with multiple ways to connect them.
For now, we're just going to touch on the main types of rigid piping you'll see in residential situations in the US. At this point, you'll most often come across rigid piping in Nominal (rigid copper piping falls under this), IPS (PVC, galvanized and stainless steel all fall under this) and CTS (Copper Tubing Size - soft copper tubing and some styles of CPVC fall under this).
Most rigid pipe is identified by IPS type sizing. PVC actually makes this pretty easy as it often has its size printed on the side! But if you're not working with PVC pipe where you can see the size, or if you're not familiar with IPS sizing, the easiest way to identify what size pipe you have so you can find the right fittings is to use a chart. (Like this one right here!) Once you have the pipe size, simply select the fittings of the same "size". For example: 1/2" IPS fittings are for 1/2" IPS pipe.
CTS matches the outside diameter of "nominal" copper pipe and tube, though the actual wall thickness might vary by pipe material. You can use a measuring tape for these types of pipe since the outside diameter is always 1/8" larger than the "size" of the pipe. Here's an example: 1/2" CPVC tubing has an outside diameter of 5/8". Again, select fittings based on the size of the pipe you're working with. Pretty easy so far, right?
Copper pipe used in residential jobs (types K, L, M, & DWV) also have an actual outside diameter that is always 1/8-inch larger than the size designation.
Copper tube for air-conditioning and refrigeration field service (ACR) is designated by actual outside diameter. Compression fittings are used fairly often for this since their size designations are for the outer diameter of the tubing they'll fit. Copper fittings are designed for nominal pipe, so to use them for OD sized copper tubing you'll need to calculate the outer diameter of the pipe they're intended for. Sound tricky? Need an example? First, remember nominal pipe has an outer diameter 1/8" larger than its size designation. So, if you need a fitting for 5/8" OD copper tubing, you'll look for 1/2" nominal copper fittings since 1/2" nominal pipe has an outer diameter of 5/8". Got it now?
All right, now that you've got the mystery of the pipe and fittings all worked out, get out there and get working on those projects with confidence!
Ready to shop for your fittings?