The French catheter scale is commonly used to measure the outside diameter of cylindrical medical instruments including catheters. On the French system, the diameter of the catheter can be determined by dividing the French size by pi, thus an increasing French size corresponds with a larger diameter catheter. However, this is contrary to needle-guage size, where the diameter is 1/guage inches, thus the larger the guage 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.

See also

• French catheter scale chart