The French scale or French gauge system is commonly used to measure the size of a catheter. It is most often abbreviated as Fr, but can often abbreviated as Fg, FR or F. It may also be abbreviated as CH or Ch (for Charrière, its inventor) in French speaking countries.
A catheter of 1 French has a diameter of ⅓ mm, and therefore the diameter of a round catheter in millimeters 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 3 mm.
An increasing French size corresponds to a larger diameter catheter. This is contrary to needle-gauge size, where an increasing gauge corresponds to a smaller diameter catheter.
The french size is a measure of external diameter of the catheter (not internal drainage channel). So, if a 2 way catheter of eg. 20 Fr is compared to a 20 Fr 3 way catheter then they both have same external diameter but 2 way catheter will contain larger drainage channel than 3 way. 3 way catheters accommodate an extra channel for irrigation in the 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.