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Endotracheal Tube Size Calculator

Selecting the correct endotracheal tube diameter is critical for safe airway management across all ages. This calculator applies evidence-based formulas to determine uncuffed and cuffed tube sizes, plus insertion depth. Anaesthetists, intensivists, and emergency physicians rely on accurate sizing to minimise airway trauma, ensure adequate ventilation, and reduce complications like vocal cord injury or tube migration.

Last updated: May 14, 2026

Creators Adena Benn, BSc

Reviewers Łucja Zaborowska and Małgorzata Koperska

8,386 people find this calculator helpful

Understanding Endotracheal Intubation

Endotracheal intubation involves passing a hollow tube directly into the trachea to secure and protect the airway during anaesthesia, mechanical ventilation, or critical care. The tube bypasses natural upper airway resistance and eliminates the air leak inherent to face-mask ventilation, where significant gas escapes around the seal or enters the oesophagus instead of the lungs.

The procedure requires visualisation of the vocal cords using a laryngoscope, careful tube advancement past the cords, and confirmation of correct positioning. Success depends on proper tube sizing: a tube that's too large causes sore throat, dental trauma, and permanent vocal cord damage; one that's too small provides inadequate ventilation and increases work of breathing.

ET Tube Sizing Formulas

For paediatric patients aged 1–12 years, Cole's formula provides a practical starting point. Adult and infant sizes follow standard reference ranges. All calculations yield internal diameter (ID) in millimetres.

Uncuffed ETT size (mm ID) = (Age ÷ 4) + 4

Cuffed ETT size (mm ID) = (Age ÷ 4) + 3

Insertion depth (cm) = (Age ÷ 2) + 12

Age — Patient age in years
Uncuffed ETT size — Internal diameter of an uncuffed endotracheal tube in millimetres
Cuffed ETT size — Internal diameter of a cuffed endotracheal tube in millimetres; typically 0.5–1 mm smaller than uncuffed to account for cuff expansion
Insertion depth — Distance from the teeth to the tube's tip at the carina, measured in centimetres

Cuffed Versus Uncuffed Tubes

Uncuffed tubes suit neonates and very small infants (under 3 kg) because their airways are naturally narrow and flexible. Cuffed tubes, fitted with an inflatable balloon, are preferred in older children and adults: they seal the airway completely, preventing aspiration of gastric contents, blood, or saliva, and they stabilise the tube during patient movement or suctioning.

Cuff pressure must be monitored—typically maintained at 20–30 cm H₂O—to balance airway protection against mucosal ischaemia and tracheal stenosis. A stylet (stiffening wire) inserted into the tube can ease advancement and allow shaping to the patient's anatomy before removal.

Why Precise Sizing Matters

Oversized tubes cause immediate pain, bleeding gums, and sore throat. Prolonged use risks permanent vocal cord paralysis, hoarseness, and laryngospasm—a potentially fatal reflex contraction of the vocal folds that can occur hours after extubation.

Undersized tubes leak gas, necessitating higher ventilation pressures, and may kink or obstruct easily. They also increase airway resistance, elevating work of breathing and risk of hypoventilation.

Factors such as body habitus, airway anatomy, and pathology (obesity, pregnancy, trauma) often require deviation from formula estimates. Experience and clinical judgment remain essential.

Clinical Tips and Considerations

Tube sizing formulas provide starting estimates, not absolute requirements.

Account for individual variation — Cole's formula assumes average body proportions. Overweight or tall children may need a size larger than predicted; those with micrognatia, micrognathia, or tracheal stenosis may require smaller tubes. Always have backup sizes (±0.5 mm) immediately available.
Confirm position every time — Insert the tube to the calculated depth, then confirm with capnography (end-tidal CO₂), auscultation for bilateral breath sounds, and chest X-ray if intubated beyond hours. Tubes can migrate with head movement or suctioning, compromising ventilation or risking right mainstem intubation.
Monitor cuff pressure carefully — Excessive cuff pressure (>30 cm H₂O) impairs tracheal blood flow and risks stenosis or tracheomalacia. Too little pressure allows air leak and aspiration. Use a cuff manometer; never estimate by feel.
Plan for emergence — Before extubating, ensure cuff is deflated, patient is alert and breathing spontaneously, and upper airway reflexes are intact. Laryngeal oedema from prolonged intubation may delay safe removal; have rescue equipment (difficult airway cart, fibreoptic scope) nearby.

Frequently Asked Questions

What is the difference between a cuffed and uncuffed endotracheal tube?

Uncuffed tubes lack an inflatable balloon and pass through the cords with minimal mucosal contact, making them ideal for neonates whose narrow subglottic regions tolerate even small cuffs poorly. Cuffed tubes feature a sealed balloon that, when inflated, prevents aspiration, stabilises the tube, and allows positive-pressure ventilation without leakage. Cuffed tubes are standard for patients over 3 kg and adults.

How is insertion depth calculated for an endotracheal tube?

Insertion depth is measured from the patient's teeth to the tip of the tube at the level of the carina (where the trachea splits into the main bronchi). The formula (Age ÷ 2) + 12 cm provides an estimate for paediatric patients; adults typically range 21–24 cm depending on height and sex. Always confirm with capnography and bilateral breath sounds; the tube should lie midway between the cords and carina to avoid mainstem intubation or accidental extubation.

Can Cole's formula be used for all paediatric patients?

Cole's formula applies best to children aged 1–12 years with normal body proportions. Neonates and infants under 1 year require age-specific sizes (typically 3.0–3.5 mm uncuffed). Adolescents and adults transition to adult sizes (typically 7.5–8.5 mm cuffed). Patients with unusual anatomy—extreme obesity, short stature, airway pathology—may deviate significantly; always individualise selection and have alternate sizes ready.

What happens if an endotracheal tube is too large?

An oversized tube creates high wall contact pressure, causing immediate throat pain, dental trauma, and gum bleeding. Over hours, persistent pressure damages the mucosa and underlying structures: vocal cord erosion leads to hoarseness and permanent voice change; laryngospasm may develop, risking complete airway obstruction post-extubation. Tracheal stenosis—narrowing from scarring—can emerge weeks later and may require surgical intervention.

How do you confirm correct endotracheal tube position?

First, visualise tube passage through the cords during intubation. Then use capnography (waveform and end-tidal CO₂ >35 mmHg confirms lung ventilation). Auscultate both lungs and the epigastrium: breath sounds should be equal bilaterally and absent over the stomach. Finally, obtain a chest X-ray to verify the tube sits 1–2 cm above the carina. Tube position should be reassessed after patient movement or if ventilation suddenly worsens.

Why does cuff pressure matter, and what is the safe range?

The cuff seals the airway and prevents aspiration, but excessive pressure (>30 cm H₂O) cuts off mucosal blood supply, causing ischaemic necrosis, stenosis, and tracheomalacia. Insufficient pressure (<15 cm H₂O) allows leakage and aspiration. Safe practice uses a cuff manometer to maintain 20–30 cm H₂O, with regular checks every 4–8 hours. In awake patients, cuff pressure should be just enough to stop air leak during positive-pressure ventilation.

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