Well, I just checked my 13/18/24mm engines with a dial caliper, and they're pretty much dead on the mm measurement. BT IDs seem to be a few thou over. I don't have anything bigger than 24mm, but the 29mm and 38mm BTs at totubular.com are also oversized.
Yes, to get inches, divide by 25.4.
29mm would be 1.142, 24mm is .945 and 18mm is .709, 38mm is 1.496
for BT sizes, (id&od) go to https://www.totubular.com/ and print the page and pin it up on the wall.