A common bile duct (CBD) measuring up to 5 mm is considered normal in most patients under age 60. Some references allow for up to 6 mm, especially post-cholecystectomy or in older individuals. Significant dilation (suggestive of obstruction) typically exceeds these measurements.

According to Rumack’s Diagnostic Ultrasound:

“The normal common bile duct measures up to 5–6 mm, with slight increases considered normal after cholecystectomy or with advancing age.”