French catheter scale

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The French catheter scale is commonly used to measure the outside circumference (= pi x diameter) of cylindrical medical instruments including catheters. In the French system, the diameter in millimeters of the catheter can be determined by dividing the French size by 3 (which can be thought of as an estimate of pi), thus an increasing French size corresponds with a larger diameter catheter. The following equations summarize the relationships:

D = F/(3 mm) = C/pi
F = C*(3 mm)/pi

For example, if the French size is 10, the diameter is 3.3 mm.

This is contrary to needle-gauge size, where the diameter is 1/gauge inches; thus the larger the gauge needle, the more narrow the bore of the needle. The Stubs Iron Wire Gauge system is also commonly used in a medical setting and is in fact more common for measuring needles, even though many find the Stubs system to be more confusing because the scale is non-linear and inversely proportional.

[edit] See also

• French catheter scale chart