Port size rpm performer manifold question

mopowers said
Listen to @TT5.9mag . I just measured the RPM manifold that's on my shelf. Trust me. Pipe is measured by ID of the pipe itself, not by the thread dimensions. NPT is measured completely different than standard thread pitch. 5/8" NPT is HUGE.

I’m just trying to learn not about not trusting, trying to understand. if I measure hole at 1/2 in how do I know what size npt it is?