# French catheter scale

The French scale or French gauge 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.[1]

The French size is three times the diameter in millimeters. Thus, the French size is roughly equivalent to the circumference of a circular catheter; the true circumference being slightly larger (${\displaystyle {\text{circumference = diameter}}\times \pi \approx {\text{diameter}}\times 3.141592654}$).

A round catheter of 1 French has an external diameter of 13 mm,[2] and therefore the diameter of a round catheter in millimetres can be determined by dividing the French size by 3:

1 mm = 3 Fr
1 Fr = 1/3 mm

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.[3]

## Size correspondence

French catheter scale
French
gauge
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