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

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)
"Tubing" is measured by the OD fairly accurately.
1/2" copper PIPE is about 5/8 OD. The ID will vary by the
"schedule/type" of the pipe. Thicker walls result in a smaller ID. The
OD has to be the same so the fittings work.