The inflatable cuff on the ET tube functions to seal the airway against oropharyngeal contents and prevent air leaks during positive pressure ventilation. The exact volume of air will vary, but should be just enough to prevent air leaks around the tube. Note: prolonged over-inflation of the cuff can cause pressure necrosis of the tracheal mucosa.

Note correct technique:

While securing the ET tube with one hand, inflate the cuff with 5-10 cc's of air. The volume of air can be adjusted later when the airway is secured.