It has been proposed that coronary artery calcium (CAC) can be used to estimate arterial age in adults. Supporting this concept is that chronologic age, as used in cardiovascular risk assessment, is a surrogate for atherosclerotic burden. This measure can provide patients with a more understandable version of their CAC scores (e.g., "You are 55 years old, but your arteries are more consistent with an arterial age of 65 years"). The aim of this study was to describe a method of calculating arterial age by equating estimated coronary heart disease (CHD) risk for observed age and CAC. Arterial age is then the risk equivalent of CAC. Data from the Multi-Ethnic Study of Atherosclerosis (MESA), a cohort study of 6,814 participants free of clinical cardiovascular disease and followed for an average of 4 years, were used. Estimated arterial age was obtained as a simple linear function of log-transformed CAC. In a model for incident CHD risk controlling for age and arterial age, only arterial age was significant, indicating that observed age does not provide additional information after controlling for arterial age. Framingham risk calculated using this arterial age was more predictive of short-term incident coronary events than Framingham risk on the basis of observed age (area under the receiver-operating characteristic curve 0.75 for Framingham risk on the basis of observed age and 0.79 using arterial age, p = 0.006). In conclusion, arterial age provides a convenient transformation of CAC from Agatston units to a scale more easily appreciated by patients and treating physicians.