French Size at a Glance
, commonly abbreviated Fr, describes the nominal outside diameter of a catheter or other round tubular medical device. A larger French number corresponds to a larger outside diameter.
Core relationship
1 Fr = ⅓ mm OD
Common example
6 Fr = 2 mm OD
What Does French Size Measure?
The French scale describes the outside diameter (OD) of a catheter or other round tubular device.
For example, a 6 Fr catheter has a nominal outside diameter of 2 mm, while a 9 Fr catheter has a nominal outside diameter of 3 mm.
Important distinction
French size alone does not define inner diameter, wall thickness, lumen configuration, or manufacturing tolerance.
The inner diameter depends on additional design characteristics such as wall thickness, lumen count, lumen geometry, liner construction, reinforcement, and other elements of the component architecture.
How Do You Convert French Size to Millimeters?
Because three French units equal one millimeter of outside diameter, the conversion is linear.
French → millimeters
mm = Fr / 3
Millimeters → French
Fr = mm × 3
6 Fr / 3 = 2 mm OD
2 mm × 3 = 6 Fr
A 7 Fr component has a nominal outside diameter of approximately 2.333 mm.
Common French Size Examples
| French Size | Outside Diameter (mm) |
|---|---|
| 3 Fr | 1.000 |
| 4 Fr | 1.333 |
| 5 Fr | 1.667 |
| 6 Fr | 2.000 |
| 7 Fr | 2.333 |
| 8 Fr | 2.667 |
| 9 Fr | 3.000 |
| 10 Fr | 3.333 |
| 12 Fr | 4.000 |
These values represent dimensional conversion, not recommended sizes for a particular medical application.
Does French Size Tell You the Inner Diameter?
No. French size identifies outside diameter. Two catheter or tubing constructions can share the same French size while having different inner dimensions.
Differences may result from:
- wall thickness
- liner thickness
- reinforcement
- number of lumens
- lumen shape
- layer construction
For multi-lumen tubing, the available internal area may be divided across multiple channels with different sizes or geometries.
Is French Size the Same as Gauge?
No. French size and gauge are different measurement systems and should not be used interchangeably.
With French sizing, a larger number corresponds to a larger outside diameter. Other gauge systems used for medical devices, such as needle gauge systems, may use different relationships between the gauge number and physical diameter.
French sizing may also appear as Fr, F, or Ch/CH, with Ch referring to the Charrière system.
Why Does French Size Matter in Catheter Development?
French sizing provides a familiar way to communicate overall outside diameter, but it represents only one part of a dimensional specification.
During catheter and medical-tubing development, teams may also need to define inner diameter, wall thickness, lumen geometry, material construction, reinforcement, dimensional tolerances, and interfaces with adjacent components. Those requirements must be considered together because changes to one part of the construction can influence other dimensional and manufacturing decisions.
French Size Within a Tubing Specification
An extrusion or catheter specification may consider:
- outside diameter (OD)
- inner diameter (ID)
- wall thickness
- number of lumens
- lumen geometry
- concentricity where applicable
- material or layer construction
- tolerance requirements
Technical reference
A 9 Fr shaft has a nominal outside diameter of 3 mm. That value alone does not identify its inner diameter, wall thickness, lumen configuration, material construction, or manufacturing tolerance.
Related Resources
Medical Tubing ArchitecturePlace outside diameter within the complete lumen, wall, material, and component architecture.
Single-Lumen Medical TubingConnect OD with ID and wall relationships in a single-channel construction.
French to Millimeters Converter →Calculate French size and millimeter outside diameter in either direction.
Knowledge Base →Return to the technical reference library.
Precision Extrusion →Explore precision medical tubing and complex extrusion capabilities.
Technical Reference
This Knowledge Base entry is intended for educational engineering reference. Dimensional requirements, materials, tolerances, and device specifications should be evaluated within the context of the specific medical-device program.