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1/2" pipe refers to the internal diameter, 15mm pipe is the external
diameter. The same olives or solder fittings will probably fit both.

I never understood why the change from imperial to metric was accompanied by
a change in measuring internal versus external diameter of pipe. Or at
least, why imperial denoted the inside diameter of the pipe.

You never fit anything inside a copper pipe: all soldered and compression
fittings go *outside* the pipe. So it makes more sense, logically, to
measure the outside diameter of the pipe, and the inside diameter of the
solder/compression fittings into which the pipe fits (those two measurements
should be virtually the same). It explains why 1/2" copper pipe wasn't
replaced by 12 or 13 mm, but 15 mm pipe - to allow for the thickness of the
wall.

But hey, this is the imperial system we were talking about: common sense and
logic (eg using the base (10) that we are taught to count in), and having
*one* definition of a unit (instead of ounces troy, avoirdupois and
apothecaries, and instead of having UK and US pints and gallons) don't apply
to imperial. ;-)


I didn't realise that solder and compression fittings for 1/2" (nominal OD
15.9 mm) and 3/4" (nominal OD 22.2 mm) would sometimes accept 15 and 22 mm
pipe respectively. A difference of 0.9 mm for 15 mm pipe is quite a large
amount for an olive or solder joint to bridge.

My parents renovated an old cottage in the mid 1970s. I remember dad having
to take out all the old 1/2" pipe and start again because he found that he
couldn't get a water-tight seal between 1/2" pipe and 15 mm fittings, when
he needed to install additional pipe - eg when the bathroom was renovated.
After he'd done all the work, he found a specialist plumber that sold
1/2"-to-15 mm and 3/4"-to-22 mm converters (I can't remember whether they
were compression or solder) which would have allowed him to continue using
the old pipe but connect new 15 or 22 mm pipe to it.