French catheter scale

Thus, the outer diameter of a catheter in millimeters can be calculated by dividing the French size by 3.

[2] For example, a catheter with a French size of 9 would have an outer diameter of approximately 3 mm.

While the French scale aligns closely with the metric system, it introduces redundancy and the potential for rounding errors.

Unlike the Birmingham gauge, where larger gauge numbers indicate smaller diameters, an increasing French size corresponds to a larger outer diameter.

The French gauge system was devised by Joseph-Frédéric-Benoît Charrière, a 19th-century Parisian surgical instrument maker.