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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 Pi (Estimates of Pi are not usually adequate if one wishes to be accurate), 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 3.18 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.

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See also

See also

★ French catheter scale chart