BACKGROUND: Correct endotracheal intubation results in better ventilation, prevents hypoxia and its possible damages, such as brain injury, and minimizes attempts for re-intubation. Up to now, several formulas have been published to estimate nasotracheal intubation tube length. This study aims to compare the accuracy of different suggested formulas to find the one that better estimates the tube insertion distance. METHODS: This cross-sectional retrospective study was carried out in 102 (51 female, 51 male) children who underwent cardiac surgery under general anesthesia. Inclusion criteria were correct nasotracheal intubation according to the postintubation chest X-ray (CXR). The estimated tracheal tube length was calculated by four different formulas. Pearson...s correlation coefficient was used to find the correlations between the estimated length of each formula and the correct nasotracheal tube length. Also, linear regression was used to obtain a formula to estimate nasotracheal tube length by weight, height, and age. RESULTS: The formula L=3*tube size+2 had the best correlation with tube length (r ...=...0.81, Confidence Interval: 0.732...0.878, p-value < 0.001). Among demographic variables, height had the highest correlation coefficient with the tube length (r...=...0.83, Confidence Interval: 0.788...0.802, p-value < 0.001). Therefore, considering the height as an independent variable and tube length as a dependent variable, using linear regression, the following formula was achieved for determining tube length: nasotracheal tube length...=...0.1*Height+7. CONCLUSIONS: The formula L=3*tube size+2 and the new suggested formula in this study can be used to estimate nasotracheal tube length in children under 4 years old. However, these formulas are only guides and require confirmation by auscultation and CXR.