Introduction: Adequate chest compression (CC) depth is critical for effective CPR. Pediatric resuscitation guidelines recommend that CC be at least one third of the anterior-posterior (AP) chest diameter or approximately 4cm in infants and 5cm in children. We aimed to find a better indicator of CC depth that maximizes CC depth while also minimizing injury. Material & Methods: Chest CT images of patients aged 8 and younger were measured for external diameter (ED) and internal diameter (ID) at the midway of the lower half of the sternum. Compressible depth was defined as 1 cm short of ID. We determined that up to a 10% estimated risk of overcompression is acceptable and approximated a quantile regression line for the 10th percentile of compressible depth on ED. After rounding coefficients, we used its equation as a new indicator. Results: 426 images were analysed. The new indicator had a slope of 0.5 and an intercept of 1.9 cm (one fingerbreadth). Compared to one third ED, the new indicator would provide deeper CC with average difference of 1.9 mm (95% confidence interval [CI], 1.6~2.2 mm) without increasing the risk of overcompression (both 4.9%). 4/5cm CC would provide deeper CC compared to the new indicator (difference: 3.5 mm, 95% CI: 2.7~4.1 mm), however, its over-compression risk was too high (31.5%). Conclusion: CC of one half ED minus one fingerbreadth maximize CC depth without increasing overcompression in pediatric population.