It's not quite so simple, especially if you include metric (French) tubing. Example: for double-butted 531 the French outer tube diameter was 28.0mm versus 28.6, so the same thickness of tubing which would use a 27.2 seatpost for British, etc. frames would use 26.6 mm post in French DB531 tubing.
Cheers!
Bob Hanson Albuquerque, NM USA ____________________ From: Emanuel Lowi
Is there some simple way to determine from seat post size (26.8, 27.0, 27.2, etc.) what gauge of classic standard diameter butted tubing (531, SL, etc.) was used for a bike's seat tube?
Emanuel Lowi
Montreal, Quebec