I'M MAD AT AMERICAN PIPES
I have a 1/2" diameter hole. I want to put a pipe inside the hole. What size PVC pipe fits into a 1/2" hole?
Did you guess 1/2" pipe? WRONG. “1/2 inch pipe” isn’t half an inch big on the outside. It’s closer to 7/8". You might think “ah so the 1/2” refers to the inner diameter". Wrong. The inner diameter of 1/2" PVC pipe is legally required to be 0.602 inches, according to the Schedule 40 standard. Want a pipe with outer diameter 1/2"? The closest size of PVC pipe is of course 1/4" PVC pipe. But even that won’t quite work, because, 1/4" pipe has outer diameter 0.54", which is more than 0.5" and way more than the 0.25" in the name of the pipe. PIPE SIZES ARE ALL LIES
Guess how I found out? I’m building a telescope, and as part of that I’m building a mount. I bought a cheap collection of 3 pipes off the internet, but I needed a fourth, so I went to the hardware store to buy some pipe. Guess who learned a $6.99 lesson in nonsensical standards…
Chamomile comments:
Wow, this is way more infuriating than 2x4’s actually being 1.5x3.5 inches.
Porglezomp addition:
Your comment made me think “oh well i know why 2x4s are like that at least that makes sense” and then I realized that there was probably a reason for this one too, so I went and figured that out:
You see, a 2x4 is 1.5"x3.5". And I thought “well, at least that has a good reason from a century ago,” but then I realized that this probably has one of those too. So let’s look into it.
Dimensional Lumber
A 2x4 is 1.5"x3.5". So why is it named like that? Well, a century ago a 2x4 referred to a 2x4 of green lumber—it was commonly cut and shipped green. And then it would shrink as it dried, and be planed to a precise size on-site. Eventually, we got better at making and transporting dry lumber, but stuck to those post-drying actual sizes for the same nominal sizes. For compatibility. So it’s the fault of compatibility with how things were manufactured over a century ago. It made sense at the time. Nominal Pipe Size
You might think “ah so the 1/2” refers to the inner diameter". Wrong. The inner diameter of 1/2" PVC pipe is legally required to be 0.602 inches, according to the Schedule 40 standard.
Well it turns out this is also the same thing. When nominal pipe sizes were standardized, they referred to the inner diameter of the pipe, but those pipes had thicker walls. Now we build stronger pipes that have thinner walls, but the dimensions were still standardized and named for those older inner diameters, and now they’re all nonsense again.
The reason for the discrepancy for NPS 1⁄8 to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thicknesses standard at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD.
Source: https://en.wikipedia.org/wiki/Nominal_Pipe_Size#Application
This standard is from 1927 / 1939 / 1949 ?? so it is more annoying than the lumber standard, to me. Diamètre Nominal
I’M MAD AT AMERICAN PIPES
Good news! European pipes are no better! A 15mm pipe has a 21.3mm outer diameter, and the inner diameter also completely depends on the material.
Source: http://www.piping-engineering.com/nominal-pipe-size-nps-nominal-bore-nb-outside-diameter-od.html