A breathing tube, which is used to secure the airway and allow ventilation of the lungs during general anaesthesia, is inserted into the windpipe either through the nose or mouth. In children, different formulas exist to determine the appropriate size of the tube according to age, and how far it should be advanced into the airway. Head movement can alter the position of the breathing tube, making it go in or come out too far. Different types of breathing tubes may also differ in their change of position with head movement. The aim of this study is to assess the accuracy of the formulae commonly used in our institution for depth of breathing tube placement, and to measure the degree of tube displacement on head movement with different types of tubes.