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Sorry, I guess I should say that this only works if you are replacing a seatpost and can measure your old one.

There's one method I always use, which has worked so far.

Clean your seatpost, obtain a large post-it note (or some other very thin sheet of paper), and wrap the post-it note around the post. It need to overlap itself. Make sure that the long edge is exactly lined up as it wraps around itself. That way you know that the post-it note is square to the seatpost.

Now mark the overlap point with a very sharp pencil. Take the post-it off, and measure along the edge to the mark. Divide by 3.142.

That should give you a sufficiently accurate measurement of the diameter.