There is nothing half inch in a half inch pipe. One inch emt is not one inch, it is 27mm outside diameter (for some reason I know that one)

I believe 1/2" pipe is exactly the same as DN15 pipe. 1/2" and 15mm are both just nominal sizes. Calipers will only help you if you happen to know the pipe schedule.

Ironically, a lot of countries with metric system calling half inch pipe a "1/2" :-)

You might eventually end up calling 15mm pipe half-inch, depending on where the cheapest pipe can be sourced from.