Endotracheal Tube Size Formulas Based on Age
For cuffed ETTs in children over 2 years, use the formula: ID (mm) = (age in years/4) + 3.5, and for uncuffed ETTs use: ID (mm) = (age in years/4) + 4. 1
Age-Based Formulas by Patient Age Group
Infants Under 1 Year
- Cuffed tube: Use 3.0 mm ID for infants ≥3.5 kg and <1 year of age 1, 2, 3
- Uncuffed tube: Use 3.5 mm ID for infants <1 year of age 2
Children 1-2 Years
Children Over 2 Years
- Cuffed tube formula: ID (mm) = (age in years/4) + 3.5 1, 3
- Uncuffed tube formula: ID (mm) = (age in years/4) + 4 3
Important Nuances About These Formulas
The Khine formula for cuffed tubes (ID = age/4 + 3) is also supported by evidence, though it may result in a marginally higher reintubation rate compared to the (age/4 + 3.5) formula 1. The 2010 International Consensus guidelines note that when the Khine formula was studied, aggressive rounding up of age often resulted in selection of tubes 0.5 mm larger than the formula actually predicted 1.
Both cuffed and uncuffed tubes are acceptable for emergency intubation in pediatric patients, though cuffed tubes may be preferable in circumstances of poor lung compliance, high airway resistance, or large glottic air leak 1, 2, 3.
Depth of Insertion Formula
- Depth at lip (cm) = weight in kg + 6 cm 2, 3
- Alternative formula: Depth = 3 × internal diameter of the tube 2, 3
- For oral intubation in children >2 years: Depth = (age/2) + 12 3
Critical Preparation Steps
Always have tubes 0.5 mm smaller and 0.5 mm larger available at the bedside before intubation 2. If the tube meets resistance during insertion, immediately use a tube 0.5 mm smaller 1, 2. If there is no leak around the tube with the cuff deflated (for cuffed tubes), consider reintubation with a tube 0.5 mm smaller when the patient is stable 1.
Common Pitfalls to Avoid
- Cuff pressure monitoring is essential: When using cuffed tubes, maintain cuff pressure <20-25 cm H₂O to avoid airway complications 2, 3
- Age-based formulas have limitations: These formulas are validated primarily for children older than 1 year and may not always be accurate 4, 5
- Length-based resuscitation tapes are more accurate than age-based formulas for children up to 35 kg 2
- Remember that neck position affects tube depth: flexion pushes the tube deeper, extension pulls it out 2
Alternative Methods
While ultrasonographic measurement of subglottic diameter shows high correlation with optimal ETT size (98% accuracy for cuffed tubes, 96% for uncuffed tubes in one study), this requires additional equipment and expertise 6, 7. The age-based formulas remain the most practical approach for emergency situations 1.