Adults: We must select the largest diameter ETT that can be tolerated for adults. A size 7.5-mm cuffed ETT is well tolerated by most adult female patients. A size 8.0-mm cuffed ETT is well tolerated by most adult male patients. Pediatrics: An uncuffed ETT should be used for children under the age of 8 years. The formula most commonly used is: ETT size(mm) = (Age[yr] +16)/4 To estimate the depth of insertion for a child older than 2 years: Depth of insertion = 3× internal diameter of the ETT