...I don't know about the newest ones, but over the years they vary as much as .1 mm from the diameter marked on the post. It's best practice when replacing a post to measure the old one with calipers, and try to replace it with a new one you measure with calipers to make certain they are close. The best measure of a post to seat tube fit is sliding it in and feeling it make contact with the seat tube as it slides down, which is often not possible when you are buying a new one off the internet. You want to feel just the slightest bit of resistance as it slides down into the greased tube, ideally.