Objective To identify phenotypic parameters correlating with the inner tracheal diameter (ITD) and endotracheal tube size (ETS) in adult dogs and to develop a chart for ETS estimation. Methods Five-hundred and forty-four adult dogs; 100 dogs were enrolled prospectively and 444 dogs retrospectively. Different phenotypic parameters, the ITD on latero-lateral radiography and ETS were prospectively measured in dogs that underwent general anaesthesia. The parameter correlated best with the ITD was used to develop a graphic chart for ETS estimation. The accuracy of this chart was then retrospectively tested. Results In prospective cohort, the correlation between body size and body mass and ITD (r=0.85 and r=0.84) was good, and the highest correlation observed between ETS and body mass (r(s)=0.92, P<0.001). In the retrospective assessment, the mean difference between the predicted and used ETS was 0.0 (sd +/- 0.84) for mesocephalic/dolicocephalic (MDC) dogs, showing high accuracy, but for brachycephalic dogs, it was 1.3 (sd +/- 0.98). Conclusion A graphic chart for ETS selection in dogs, using body mass, was designed in this study and was demonstrated to be accurate for ETS prediction in MDC dogs but not in brachycephalic dogs.