About

In medical procedures, precision is non-negotiable. The French Scale (Charrière scale) is the global standard for denoting the external diameter of catheters, needles, and guide wires. A misunderstanding of the conversion between French (Fr) and millimeters (mm) can lead to equipment incompatibility or patient injury.

This tool provides an instant, bidirectional conversion based on the linear definition where 1 Fr equals 0.333 mm. It also includes a reference to the ISO 6009 color coding standard, helping healthcare professionals quickly identify catheter sizes visually during critical care scenarios.

Formulas

The relationship is strictly linear. The scale was designed by Joseph-Frédéric-Benoît Charrière such that the French size is the circumference in millimeters, implying the diameter is divided by Pi approximately (simplified to 3 for manufacturing standards).

Dmm = Fr3

Fr = Dmm × 3

Note: Manufacturers round the millimeters to one or two decimal places.

Reference Data

French Size (Fr)	Diameter (mm)	ISO Color Code	Common Use
3 Fr	1.0 mm	Grey (or White)	Neonatal / Pediatrics
4 Fr	1.33 mm	Red	Pediatrics
5 Fr	1.67 mm	Grey	Feeding / Aspiration
6 Fr	2.0 mm	Green	Pediatrics / Female Cath
8 Fr	2.7 mm	Blue	Adult Aspiration
10 Fr	3.3 mm	Black	Suction / Rectal
12 Fr	4.0 mm	White	Adult Urinary
14 Fr	4.7 mm	Green	Standard Adult Urinary
16 Fr	5.3 mm	Orange	Standard Adult Urinary
18 Fr	6.0 mm	Red	Post-Op / Hematuria

Frequently Asked Questions

No. The Gauge scale is inverse (larger number = smaller needle) and non-linear. The French scale is direct (larger number = larger diameter). They are not interchangeable.

The French scale strictly measures the **Outer Diameter (OD)** of the tube. The size of the internal lumen (ID) varies depending on the wall thickness of the material (silicone, latex, or PVC).

ISO 6009 standardizes color coding for hypodermic needles and some catheters to allow rapid identification in emergency settings without reading small print on the packaging.