The length of Eustachian tube is

Eustachian tube, also called auditory or pharyngotympanic tube, connects nasopharynx with the tympanic cavity. In an adult, it is about 36 mm long and runs downwards, forwards and medially from its tympanic end, forming an angle of 45deg with the horizontal. It is divided into two pas: bony, which is posterolateral, forms one-third (12 mm) of the total length and f ibrocailaginous, which is anteromedial, forms two-thirds (24 mm). The two pas meet at isthmus which is the narrowest pa of the tube . Reference-Dhingra 6/e, p57.