A) Outer diameter of the tube
B) Inner diameter of the tube
The French gauge system to measure the size of a catheter is first invented by a swiss-born French manufacturer of surgical instruments, named Charrière. Chest tubes are still referred to French sizes. The formula to comprehend the tube size in mm is easy to remember when size is referred to F.
D (mm) = Fr / 3
This Diameter (D) measurement refers to the outer diameter of the tube in conversion.
Iserson, Kenneth V. (1987). "J.-F.-B. Charrière: The man behind the "French" gauge". Journal of Emergency Medicine. 5 (6): 545–8.