Objectives: The increasing popularity of online blood alcohol concentration (BAC) calculators which estimate alcohol levels based on user-entered variables such as sex, weight, and alcohol consumption data raises concerns about their reliability and their influence on the decision to drive. The aim of this study is to assess the accuracy and usability of 36 online BAC calculators in Polish and English. Material and Methods: The calculated BAC values were compared with reference values obtained from 8 female, 8 male in both fasted and post-meal states. Blood alcohol concentration was evaluated at the predicted maximum value and at 60 min and 120 min post-consumption. Results: Significant variability was noted: mean values exceeded reference concentrations by 39–49%, and few calculators correlated with reference values. Only the BACscan, Aisko, and Website No. 3 calculators yielded results comparable to reference values under certain conditions (Passing-Bablok regression and Bland-Altman plots). Furthermore, in the present study an online survey (N = 103) found that although 73.2% of users had used an online breathalyzer, 85.4% of these did not base decisions (e.g., driving) on their results, believing them to be unreliable. Conclusions: Overall, most online BAC calculators lack sufficient precision and are unsuitable for assessing intoxication. These findings underscore the need for public education on the limitations of such tools and call for better regulation of their accuracy and presentation. Int J Occup Med Environ Health. 2026;39(3)