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Default Copper pipe sanity check, please

On Mon, 07 Jun 2021 15:03:06 GMT, (Scott Lurndal)

On Sun, 06 Jun 2021 20:47:52 -0400, Nil

I need to replace a water shut-off valve below my kitchen sink. I see
that valves come made to fit on either 1/2" or 5/8" pipe. When I
measure the copper intake pipe with my (not very accurate) caliper it
measures .66, which is a little over 5/8". I think I read somewhere
that "1/2 inch" refers to the inside diameter, and that the outside
diameter is closer to 5/8. True? No?

So, what do I have, 1/2" or 5/8" copper pipe? I'm thinking 1/2" because
it's more common, and if it were 5/8" the outside diameter might be
greater than .66".

Also, Home Depot has valves with Shark Bite connectors which don't
require compression fittings. Are those things reliable?

"Pipe" is specified by the nominal ID. (the OD remains the same for
each nominal size)

Usually, but not always. For example, CPVC vs. PVC.

CPVC is measured by O.D., which makes its sizing similar
to hard copper. PVC is measured by I.D., which makes its
sizing similar to iron pipe.

That isn't exactly true either.
CPVC has the same dimensions as the corresponding copper pipe. (1/2"
is 5/8"OD)
It works with the same shark bites

PCV has the dimensions of galvanized or black iron pipe.
If it is threaded, galvanized fittings work. (usually only found in
schedule 80)