ETT Size Calculator — Endotracheal Tube Size for Paediatric Patients

Formula

How this is calculated

For children over one year of age, the Motoyama formula (uncuffed ID = Age/4 + 4 mm) and the Khine 1997 modification (cuffed ID = Age/4 + 3.5 mm) provide a rapid estimate of the correct endotracheal tube internal diameter. These formulas exploit the near-linear relationship between age and subglottic tracheal diameter in growing children. Both have been prospectively validated across multiple studies and are recommended in PALS (Paediatric Advanced Life Support) guidelines as first-line estimates.

For neonates and premature infants, airway dimensions are determined primarily by gestational age and weight rather than postnatal age. Weight-based sizing (2.5 mm for <1 kg, 3.0 mm for 1–2 kg, 3.5 mm for 2–3.5 kg, 3.5–4.0 mm for >3.5 kg) is the standard approach in neonatal intensive care. Uncuffed tubes are typically used in neonates because the narrow circular cricoid ring acts as a natural seal; cuffed tubes are increasingly preferred in older children to improve ventilator seal and reduce aspiration risk.

Insertion depth at the lips is conventionally estimated as three times the tube ID (cm), giving a mid-tracheal position in most children. Nasal intubation adds approximately 2 cm. All size and depth estimates are starting points only: tube position must be confirmed with capnography, bilateral chest auscultation, and ideally fibre-optic bronchoscopy or chest X-ray in the intensive care setting. Anatomical variation, subglottic stenosis, and large-for-age or small-for-age patients require clinical reassessment.