<P> From the basic math equation C = πd, it follows that the catheter's circumference in mm is only slightly (about 4.7%) greater than the French size . Even though this relation is not - even - wrong, one could just have defined the French as 1mm / π . </P> <P> An increasing French size corresponds to a larger external diameter . This is contrary to Birmingham gauge, where an increasing gauge corresponds to a smaller diameter needle . </P> <P> The French size is a measure of the outer diameter of a catheter (not internal drainage channel, or inner diameter). So, for example, if a two - way catheter of 20 Fr is compared to a 20 Fr three - way catheter, they both have the same external diameter but the two - way catheter will have a larger drainage channel than the three - way . Three - way catheters accommodate an extra channel for irrigation within a similar external diameter . </P> <P> The French gauge was devised by Joseph - Frédéric - Benoît Charrière, a 19th - century Parisian maker of surgical instruments, who defined the "diameter times 3" relationship . </P>

How many mm are in an i ch