French catheter scale
The French size is three times the diameter in millimeters. A round catheter of 1 French has an external diameter of 1⁄3 mm, and therefore the diameter of a round catheter in millimetres can be determined by dividing the French size by 3:
D (mm) = Fr / 3
Fr = D (mm) * 3
For example, if the French size is 9, the diameter is 9/3 = 3.0; mm.
From the basic math equation C = πd, it follows that the catheter’s circumference in mm is only slightly (about 4.7%) greater than the French size.
An increasing French size corresponds to a larger external diameter. This is contrary to needle-gauge size, where an increasing gauge corresponds to a smaller diameter needle.
The French size is a measure of the external diameter of a catheter (not internal drainage channel). So, for example, if a two-way catheter of 20 Fr is compared to a 20 Fr three-way catheter, they both have the same external diameter but the two-way catheter will have a larger drainage channel than the three-way. Three-way catheters accommodate an extra channel for irrigation within a similar external diameter.
The French gauge was devised by Joseph-Frédéric-Benoît Charrière, a 19th-century Parisian maker of surgical instruments, who defined the “diameter times 3” relationship.