OBJECTIVE To determine the average value of the diameter of the common bile duct by age group in adult patients without bile duct pathology. METHODS A cross-sectional, observational, descriptive and prospective study in adults without bile duct pathology who underwent abdominal ultrasound. The internal diameter of the common bile duct was measured after questioning. In the descriptive analysis, absolute frequencies, percentages, mean and standard deviation were used in the inferential Student's t test and Pearson's correlation. RESULTS 608 patients without bile duct pathology were included, the mean diameter of the common bile duct was 4.04 ± 1.11 mm; with a minimum of 1.9 mm and a maximum of 8.8 mm. The mean for the population under 30 years was 3.8 ± 0.87, for the 30-39 years group it was 4.27 ± 0.89, for the 40-49 years group it was 4.39 ± 0.83, for the 50-59 years 4.86 ± 1, for the 60-69 years group it was 5.4 ± 0.95, and for those over 70 years it was 6.9 ± 0.8. CONCLUSIONS The diameter of the common bile duct in adult patients without biliary pathology is greater at older age.