If the jacket is off the rack (or off the peg as they say in the UK) and made in the UK, you will wear at least one chest size (2'') larger than it is labelled. This seems to be true with Argyles, Prince Charlies, the various doublets and every other kilt jacket I have come across.

I don't know why this is. Perhaps it's that we Americans are fatter than our British counterparts. Or perhaps inches are larger over there.