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I'm really confused. I've only ever used metric in my life. Recently I'm trying to build a IOT device and there is plumbing and water involved.

I was looking for a 19mm fitting, but on Amazon and other sites it's always advertised in inches. John Guest 3/4" BSP x 1/4" Push Fit Tap Adapter

I did the conversion and 3/4 inch, according to my calculation, should be around 19mm. But whenever I order these fittings they always send me a larger size. The resulting size is always around 25mm when I measure it.

To me it seems like 3/4 inch doesn't actually mean 3 quarters of an inch?

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    $\begingroup$ For UK plumbing, "3/4 inch" is the internal diameter. The external diameter of "3/4 inch pipe" IS close to 25mm (in fact, a bit smaller). I suspect you have got the product you ordered, but you misunderstood what size you should have ordered. $\endgroup$
    – alephzero
    Jan 24 '19 at 17:40
  • $\begingroup$ @alephzero going by what you said and after doing quite a bit of research today on the topic it seems that this is correct. I suspect that the diameter they mentioned was the actual diameter of the pipe, and not the diameter of the threaded fixture $\endgroup$
    – Andre
    Jan 24 '19 at 20:18
  • $\begingroup$ That sounds correct. A "3/4 in fitting" fits properly onto a "3/4 in pipe", and similarly for 1/2 in, 15mm, 22mm ,etc. Only the manufacturers need to know exactly what size the various parts are. So far as the users are concerned "3/4 in" is just a name, not a measurement. $\endgroup$
    – alephzero
    Jan 24 '19 at 22:31
  • $\begingroup$ It might be worth adding that in the UK the "inch" sizes only continue to be made for compatibility with the pipework in old buildings. Inches have not been legal dimensions for any trade purposes in the UK since the 1970s. Any buildings after that date will most probably have "metric sized" pipework - usually 15mm and 22mm for domestic plumbing. $\endgroup$
    – alephzero
    Jan 24 '19 at 22:41
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Pipes and pipe fittings in the US are mostly NPT (National Pipe Thread (or Tapered, depending upon who you ask). (Check out the NPT Wikipedia article.) A 3/4 inch NPT fitting is not 3/4 inch anywhere, except in name. The inside diameter of a 3/4 inch nominal pipe is slightly more than 3/4 inch but if you want to drill to tap a 3/4 inch NPT, you will need to drill a hole 59/64 in diameter, which sounds like the one inch you were seeing.

But that is not what you were asking.

Since you are looking at Amazon, UK at a BSP, British Standard Pipe, link, I expect you are not US based. BSP is another standard, incompatible both with NTP and Metric.

What I suggest is going down to your local hardware store and seeing what they have most of and going with that. If you need anything subtle, you will need to go to a plumbing supply house or Amazon.

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  • $\begingroup$ While everyone here gave good answers, I'm going to mark this one as solving my issue because it's got some valuable info. In the end the actual issue was my own misunderstanding of the seller's product description, as pointed out by @alepzero on my original post. $\endgroup$
    – Andre
    Jan 24 '19 at 20:21
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Yes it is BUT BSP (and pipe fittings in general) fittings are specified by the nominal bore rather than the thread diameter, which can be confusing if you are used to dealing with metric fasteners.

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you are right 3/4" means $ 25.4mm/inch* \frac{3}{4}= 19.05mm $

If there is any mismatch, it is there fault.

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Plumbing fittings from good plumbing suppliers are available in metric and imperial sizes.

You need to use either imperial or metric all the way through, but there are some copper fittings that are available that are conversion fittings that will convert from imperial to metric...

These fittings are usually compression or solder fittings.

And, yes, I have faced the same issue on my parent's house with old imperial sized pipes and I now keep the conversion fittings to convert to the metric sizes ie 1/2" pipe is now 15mm etc.

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    $\begingroup$ Don't forget that the old UK "imperial" plumbing sizes referred to the INTERNAL diameter of the pipes, but metric sizes refer to the EXTERNAL diameter. For example "1/2 inch" pipe is almost exactly the same physical size as "15mm," and the two sizes can be joined WITHOUT a special adapter - a 15mm coupler or a 1/2 in coupler both work OK. That is not what you might expect if you know that 1 inch = 25.4 mm! $\endgroup$
    – alephzero
    Jan 24 '19 at 17:22
  • $\begingroup$ @alephzero but the solder fittings don't fit... you need the adapter fittings... it may be only a small amount... but they don't go in... At least the old imperial pipes in my parents house cause real issues... As I said I keep the adapter fittings ... $\endgroup$
    – Solar Mike
    Jan 24 '19 at 17:26
  • $\begingroup$ That depends how much elbow grease you use cleaning the bigger diameter pipe with a coarse file before you solder it ;) You might have problems if you use Yorkshire joints, though - they are pre-loaded with solder, so the tolerances are smaller. $\endgroup$
    – alephzero
    Jan 24 '19 at 17:33
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No, as noted by@ user 1683793 , 3/4 inch is the NAME of a size of ASTM pipe ( not the size) , very close to API pipe. Any 3/4 pipe will thread to any other whether is is Schedule 10 or Sch 120 , stainless or copper, etc. ; only the wall thickness and inner diameter change. Most often Sch 40 is used for domestic service. And tubing has nothing to do with pipe other than it may look similar. I am not familiar with pipe threaded to ISO,BS, DIN, JIS, GOST, etc. Although I suspect CSA ( Canadian) is very close to ASTM.

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