Although computed tomography (CT) has played an important role in evaluation of the thoracic aorta, no standards for aortic dimensions exist. To establish the range of normal variation of aortic diameters, a retrospective study of 102 chest CT studies in adults without clinical evidence of hypertension, diabetes, cardiovascular disease, or renal disease was performed. The coronal aortic diameter was measured at three levels: just beneath the aortic arch, just above the aortic valve, and at the level of the diaphragm. These measurements showed substantial variation according to age, sex, and thoracic vertebral body width. The ratio between the ascending and descending limbs of the aorta varied markedly with age; younger individuals had significantly higher ratios than older age groups. Knowledge of these values allows more precise CT evaluation in suspected aortic disease, specifically reducing the potential for "overinterpretation" of the normal, but prominent, ascending aortic root.