Endotracheal Tube Size Calculator (children 1–10 years, cuffed & uncuffed)

Understanding the endotracheal tube size calculator — concept and principle

This calculator gives a starting estimate for the internal diameter and insertion depth of a breathing (endotracheal) tube for a child aged 1 to 10 years, for both cuffed and uncuffed tubes. It is an equipment-sizing reference for trained clinicians in anaesthesia, emergency and intensive care.

Tube size from age: with the default inputs the result is 13.5 cm (depth at lips = 3 × 4.5).

How it is calculated

Uncuffed internal diameter (mm) = age ÷ 4 + 4

Cuffed internal diameter (mm) = age ÷ 4 + 3.5

Insertion depth at the lips (cm) = 3 × internal diameter

Age is in years; decimals are allowed.
Tubes are made in 0.5 mm steps, so the calculated value is rounded to the nearest available size in practice.

The principle

A child's airway grows roughly in step with age, so a straight-line formula gives a usable first guess. The cuffed tube is half a size smaller because the inflated cuff, not the tube wall, seals the airway, which lowers the risk of pressure on the narrow part of the trachea. The depth rule "3 × ID" reflects that tube size and trachea length both scale with the child's size.

Worked example

Default input: age 4 years.

Cuffed ID = 4 ÷ 4 + 3.5 = 4.5 mm.
Uncuffed ID = 4 ÷ 4 + 4 = 5 mm.
Depth, cuffed = 3 × 4.5 = 13.5 cm.
Depth, uncuffed = 3 × 5 = 15 cm.

Each point is the calculator re-run with only “Child’s age (years)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Child’s age (years)	Cuffed tube internal diameter (mm)
1	3.75
2	4
3	4.25
4	4.5
5	4.75
6	5
8	5.5

How to read the result

Treat the result as the middle size and have tubes 0.5 mm smaller and larger ready. A tube that is too large may injure the airway; one that is too small leaks or raises resistance. The insertion depth is an estimate; correct position must be confirmed by clinical signs, capnography and, when needed, a chest X-ray.

Common mistakes

Using the uncuffed formula for a cuffed tube, which gives a tube half a size too big.
Ignoring body size. Children who are small or large for their age need adjustment; a length-based tape is often more accurate in emergencies.
Using the formula for infants under 1 year, where different sizes apply.
Relying on depth alone without confirming placement.

Keep in mind: This is a reference for trained professionals, not a substitute for training, local protocols or clinical judgement.

Frequently asked questions

Why are cuffed tubes now common in children?

Modern low-pressure cuffs seal the airway with a smaller tube, which reduces leaks and the need to change tubes.

What does ID mean on a tube?

Internal diameter in millimetres, printed on the tube.

Is the depth measured at the lips or the nose?

The formula here gives depth at the lips for oral intubation.

Sources: MSD Manual Professional calculator, "Endotracheal tube size for children" (Khine HH et al., Anesthesiology 1997; King BR et al., Ann Emerg Med 1993) · University of Maryland UMEM Educational Pearls, "Tube sizes", 2011

Checked 9 Nov 2026. Age-based formulas uncuffed ID = age/4 + 4 and cuffed ID = age/4 + 3.5 (Khine HH et al., Anesthesiology 1997; King BR et al., Ann Emerg Med 1993) as given in the MSD Manual professional calculator (Endotracheal tube size for children); insertion depth 3 × ID as given in UMEM Educational Pearls (University of Maryland), “Tube sizes”, 2011.

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