How Endotracheal Tube (ETT) Size Is Calculated
Selecting an endotracheal tube requires balancing an internal diameter large enough for adequate ventilation and suctioning against one small enough to pass safely through the airway without causing trauma. For children roughly 1 to 15 years old, the most widely taught reference is Cole's formula, which estimates internal diameter (ID) directly from age: ID (mm) = (age ÷ 4) + 4 for an uncuffed tube, or (age ÷ 4) + 3.5 for a cuffed tube. This calculator applies that formula, plus standard reference sizing for newborns and adults, and a depth-of-insertion rule of thumb. Every result here is an educational estimate, not a prescription — actual tube selection always depends on direct clinical assessment.
How Cole's formula works
The age-based formula reflects the fact that airway diameter grows roughly linearly with age during childhood. For a 4-year-old, the uncuffed estimate is (4 ÷ 4) + 4 = 5.0mm, and the cuffed estimate is (4 ÷ 4) + 3.5 = 4.5mm — cuffed tubes are sized about 0.5mm smaller because the inflatable cuff itself takes up space within the airway. In practice, clinicians always prepare the calculated size plus one size above and one size below, since the formula is a starting estimate and actual airway anatomy varies between patients of the same age.
Insertion depth and confirming placement
A commonly used rule of thumb estimates oral insertion depth (in cm, measured at the lip) as about three times the tube's internal diameter in millimeters (depth ≈ ID × 3). A tube placed nasally typically sits about 2cm deeper than the equivalent oral depth because of the added length of the nasal passage. Insertion depth from a formula is only a starting point — final position must always be confirmed with bilateral chest auscultation, capnography (end-tidal CO2), and often a chest X-ray, since a tube placed too deep commonly enters the right mainstem bronchus.
Why newborns and adults use different conventions
Cole's formula is not intended for newborns: a term newborn typically takes a 3.0mm (cuffed) or 3.5mm (uncuffed) tube, while a preterm or very-low-birth-weight infant may need a tube as small as 2.5mm, sized from weight and gestational age rather than from age in years. At the other end of the spectrum, adult sizing is conventionally based on sex rather than a formula: adult women commonly receive a 7.0-7.5mm cuffed tube and adult men a 7.5-8.0mm cuffed tube, with cuffed tubes used almost universally in adults. This calculator reflects both conventions automatically based on the age you enter.