French and gauge medical equipments
A French (Fr) is a unit of measure of diameter that is often applied to catheters. 1 Fr is equal to one third of a millimeter. A 6 Fr catheter is therefore 2 mm in diameter. A gauge is also a unit used to measure diameter where higher gauge values are associated with smaller diameters.
Although it can be used to measure catheters, it is frequently used for needles. For example, a 21-gauge needle is approximately 0.03 inches in outer diameter, whereas an 18-gauge needle is 0.05 inches in outer diameter.