 The tip of a correctly positioned tracheal tube will be in the mid @-@ trachea , between the <unk> on an <unk> chest radiograph . The correct diameter of the tube is that which results in a small leak at a pressure of about 25 cm ( 10 in ) of water . The appropriate inner diameter for the endotracheal tube is estimated to be roughly the same diameter as the child 's little finger . The appropriate length for the endotracheal tube can be estimated by doubling the distance from the corner of the child 's mouth to the ear canal . For premature infants 2 @.@ 5 mm ( 0 @.@ 1 in ) internal diameter is an appropriate size for the tracheal tube . For infants of normal gestational age , 3 mm ( 0 @.@ 12 in ) internal diameter is an appropriate size . For normally nourished children 1 year of age and older , two formulae are used to estimate the appropriate diameter and depth for tracheal intubation . The internal diameter of the tube in mm is ( patient 's age in years + 16 ) / 4 , while the appropriate depth of insertion cm is 12 + ( patient 's age in years / 2 ) . 
