Tracheostomy Tube Size Conversion: 18 French to Millimeters
An 18 French tracheostomy tube has an external diameter of 6 millimeters.
Understanding the French Scale Conversion
The French (Fr) scale is a standardized measurement system used for medical catheters and tubes, where:
- 1 French = 0.33 millimeters (or more precisely, 1 Fr = 1/3 mm)
- 18 French = 6.0 millimeters external diameter 1
This conversion applies specifically to the outer diameter of the tube, not the internal diameter 2, 3.
Clinical Context for Tracheostomy Tube Sizing
Key Sizing Parameters
The dimensions of tracheostomy tubes include 2, 3:
- Inner diameter (ID) - determines clinical utility and airflow
- Outer diameter (OD) - what the French scale measures
- Length - must extend at least 2 cm beyond stoma and remain 1-2 cm above carina 1, 4
- Curvature - must be concentric and colinear with the trachea 1, 5
Appropriate Tube Selection Considerations
When selecting tracheostomy tube size, the diameter should 1, 4:
- Avoid damage to the tracheal wall
- Minimize work of breathing
- When possible, promote translaryngeal airflow
- Be verified with neck/chest radiographs or flexible bronchoscopy to assess proper positioning 1
Common Pitfall
Do not confuse outer diameter (measured in French) with inner diameter (measured in millimeters). A tube labeled as 18 French refers to its external diameter of 6 mm, but its internal diameter will be smaller depending on wall thickness 2, 3. The inner diameter is what determines the tube's functional capacity for ventilation and suctioning.