Management of clavicle fractures depends in part on the magnitude of shortening compared with the contralateral, intact clavicle. However, estimating prefracture length may be complicated by side-to-side asymmetry. The authors retrospectively reviewed chest radiographs using a ruler tool to measure clavicle length in 696 patients 12 to 25 years old. Absolute and relative least significant change values that represented statistically significant true biological differences were 3.54 mm and 2.44%, respectively. Length differences in which the left was greater than the right or the right was greater than the left exceeding the absolute least significant change were found in 35.2% of males and 39.8% of females. Length differences exceeding 5 mm and 10 mm were 19% and 2.6% in males and 22.6% and 0.9% in females, respectively. There was a statistically significant left bias in length difference, with a 2.1-fold and a 1.6-fold greater chance of the left being longer than the right (although usually <5 mm in actual length) in males and females, respectively. The authors concluded that small but significant left-biased length asymmetry prevailed in both sexes. Differences exceeding 10 mm were rare. Contralateral clavicles can be used to estimate prefracture length of presenting fractures with the understanding that small left-right length discrepancy is common.