there has been a special standard established for navel barbells (also called "bananabells" or "bananabars", a reference to their curved shape). the standard barbell is 1.6 mm (1/16") thick and 9.5 mm (3/8") or 11.1 mm (7/16") long and is most commonly referred to as a 14-gauge post. the silver caps on the barbell post usually measure 5mm in diameter for the upper and 8mm in diameter for the lower.