This study was performed to determine a landmark for chest compression depth for adult CPR using chest computed tomography (CT) and to evaluate the validity of the landmark. The external anteroposterior diameters (EAPDs) of each chest at the suprasternal notch (SN) and the lower half (LH) of the sternum were measured. We analyzed the differences in the EAPDs between the LH and the SN in each EAPD groups in the LH of the sternum as follows: <20.00, 20.00-21.99, 22.00-23.99, ≥24.00. We compared the differences in the EAPDs between the two points with 50 mm and the chest compression depth on simulated one-fourth external chest compressions for each EAPD groups on the LH of the sternum. The mean difference in the EAPDs between the SN and the LH was 5.16±0.91 mm. The differences in the EAPDs between the SN and the LH of the sternum with 50 mm did not indicate a significant difference. The mean one-fourth EAPD at the LH of the sternum was 5.50±0.53 mm. There was not a significant difference in the residual chest depth on one-fourth simulated chest compression for each EAPD groups on the LH of the sternum. The SN may have value as a functional landmark for chest compression depth in adult cardiopulmonary resuscitation. Our findings combined with the simulated one-fourth chest compressions were more consistent with the depth of ≥50 mm recommended by the 2010 CPR guidelines.