An accurate assessment of the biting shark size is paramount for better understanding the agonistic behavior of a species toward humans. The gray reef shark is involved in many accidental bites. Based on the capture of 35 gray sharks, we calculated the algorithm that allows the assessment of the shark size, through the interdental distance (IDD) inferred from teeth marks on human wounds. Our results show a negative allometric relationship and that IDD calculated from imprints perpetrated by the upper jaw are globally similar with those from the lower jaw, in spite of heterodonty. We applied our findings to two cases of accidental bites by this species. Both of the victims had declared that the shark length was “at least 2 m”. Based on our algorithm, the assessment of the shark TL was approx. 180 and 160 cm, respectively, which correspond to an overestimation of ≥20% by the victims.