french catheter scale french catheter scale the french scale, french gauge or charrière system is commonly used to measure the size of a catheter. it is most often abbreviated as fr, but can often be seen abbreviated as fg, fr or f. it may also be abbreviated as ch or ch (for charrière, its inventor). however, simply gauge, g or ga generally refers to birmingham gauge. the charrière is measured by the ''outer'' diameter, and is defined as 1 fr = 1/3 mm, and thus 1 mm = 3 fr; therefore the diameter of a round catheter in millimetres can be determined by dividing the french size by 3., for example, if the french size is 9, the diameter is 9/3 = 3.0; mm. it is seen that the french unit is fully redundant with the metric system but introduces potential for rounding errors. this metrication problem is further complicated by the mixed use of metric and imperial units in medical professions using catheters. 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. 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. 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. size correspondence french catheter scale frenchgauge circumference(mm) outer diameter (mm)(inches) 3 3.14 1 0.039 4 4.19 1.333 0.053 5 5.24 1.667 0.066 6 6.28 2 0.079 7 7.33 2.333 0.092 8 8.38 2.667 0.105 9 9.42 3 0.118 10 10.47 3.333 0.131 11 11.52 3.667 0.144 12 12.57 4 0.158 13 13.61 4.333 0.170 14 14.66 4.667 0.184 15 15.71 5 0.197 16 16.76 5.333 0.210 17 17.81 5.667 0.223 18 18.85 6 0.236 19 19.90 6.333 0.249 20 20.94 6.667 0.263 22 23.04 7.333 0.288 24 25.13 8 0.315 26 27.23 8.667 0.341 28 29.32 9.333 0.367 30 31.42 10 0.393 32 33.51 10.667 0.419 34 35.60 11.333 0.445