If gallbladder present, normal caliber of common bile duct (CBD) is up to 6 mm, but you can add an extra 1 mm for every decade after age 60. For example, if age 80, the CBD can measure up to 8 mm and still be normal. If the CBD is dilated, the next step is usually MRCP (noninvasive imaging test done by radiologist) and/or ERCP (invasive endoscopic study done by gastroenterologist) to rule out something obstructing the duct such as a stone (choledocholithiasis).
If gallbladder absent, CBD can go up to approximately 1 cm and still be normal.