In most clinical scenarios, left DLT is preferred for one-lung ventilation because of its anatomical ease of placement; these tubes allow separate ventilation of both lungs. If the DLT is not placed in the proper size and depth, it may result in repeated intubation attempts, airway and dental trauma, failed lung isolation, tube dislodgement, and various unwanted events such as hypoxemia. The first and most common method for correct placement of a DLT is the conventional technique, blindly advanced into the left main bronchus, and then confirmed with fiberoptic bronchoscopy (FOB). In this method, the depth at which the tube should be left before performing FOB is left to the clinician's experience. Generally, the DLT is advanced in the trachea until a slight resistance is felt. This may lead to excessive advancement of the DLT into the left main bronchus or premature resistance due to the tube tip touching the carina, causing the clinician to stop before entering the left main bronchus. Therefore, just as selecting the correct size of the DLT is crucial, correctly estimating the appropriate depth is also of great importance. For this reason, different formulas have been proposed in the literature, and new formulas are still being investigated. The patient's gender and height are determinant in selecting the appropriate size of the DLT. However, studies in the literature indicate that the accuracy of these formulas may be limited in Asian populations. Therefore, it is important to evaluate the applicability of these formulas in different populations and, if necessary, develop new formulas. In the Turkish population as well, verifying the accuracy of these formulas for determining the proper size and depth of DLT-and if needed, developing new recommendations and formulas-holds clinical importance. In this study, conducted at Ankara Atatürk Sanatorium Training and Research Hospital, the aim is to evaluate the accuracy of six different formulas available in the literature for predicting DLT depth in patients undergoing thoracic surgery. Additionally, the correlations between DLT depth and demographic parameters as well as external airway measurements (mouth opening, sternomental distance, thyromental distance, distance between the mentum and manubrio-sternal angle, distance between tragus and manubrio-sternal angle, distance between sternal angle and xiphoid process) will be analyzed. Furthermore, challenges during DLT application, malposition rates and types, and complications will be assessed. The primary objective of this study is to evaluate, in patients undergoing thoracic surgery at Ankara Atatürk Sanatorium Training and Research Hospital, how accurate and applicable six different formulas defined in the literature are for predicting the placement depth of the DLT. If the existing formulas are insufficient, the aim is to develop a new formula.