The main objective of this study was to develop a brain injury criterion based on FE computation of axon elongation. To achieve this objective a state‐of‐the‐art finite element head model with enhanced brain and skull material laws, was used for numerical computation of real world head traumas. No less than 109 well‐ documented traumatic brain injury cases were simulated and axon elongation computed to derive brain injury tolerance curve. Based on statistical analysis of different intracerebral parameters (Von Mises stress, Von Mises strain and axonal strain) it was shown that axonal strain was the appropriate candidate parameter to predict diffuse axonal injury. The proposed brain injury tolerance limit for a 50% risk of diffuse axonal injury has been established at about 15% of axonal strain. Further head kinematics based metric:​ Head Injury Criterion has also been computed for all head trauma cases to compare its injury prediction capability with the axon strain based metric. This study provides a key step for a realistic injury metric for diffuse axonal injury.