Fr → mm
mm = Fr ÷ 3
01 / French Size Converter
French to Millimeters Converter
Convert French catheter and tubing size to outside diameter in millimeters—or millimeters back to French size.
Outside-diameter conversion
6 Fr
=2.000 mm OD
1 Fr = 1/3 mm outside diameter
02 / Conversion
French size ↔ millimeters
Fr
Enter a positive decimal value. Conversion updates immediately.
Outside diameter
mm OD
mm = Fr ÷ 3 · Nominal outside-diameter equivalence
03 / Outside Diameter
French size describes the outside diameter.
The inner lumen is shown only as visual context. French size does not define its diameter or the tube wall.
Illustrative example
6 Fr = 2.000 mm OD
Illustration is conceptual and not physically to scale.
04 / Technical Reference
Understand the sizing system behind the conversion.
French size is commonly used to describe catheter and medical-tubing outside diameter. One French unit corresponds to one-third of a millimeter.
Reference / Catheter Technologies
French Sizing for Catheter Tubing
OD relationshipConversion contextEngineering reference
Read the French Sizing Reference →05 / Conversion Method
Two directions. One dimensional relationship.
mm → Fr
Fr = mm × 3
The conversion defines nominal outside-diameter equivalence only. It does not define inner diameter, wall thickness, lumen configuration, or manufacturing tolerance.
3 Fr
1.000 mm
4 Fr
1.333 mm
5 Fr
1.667 mm
6 Fr
2.000 mm
7 Fr
2.333 mm
8 Fr
2.667 mm
9 Fr
3.000 mm
12 Fr
4.000 mm
06 / Related Engineering Tools
Continue with another engineering question.
Engineering + Program Support
Need support beyond
the conversion?
Cordica engineering and manufacturing teams can help connect dimensional, material, component, and production requirements around the broader device program.