French catheter scale

The French size is three times the diameter in millimeters. A round catheter of 1 French has an external diameter of ¹⁄₃ mm, and therefore the diameter of a round catheter in millimetres can be determined by dividing the French size by 3:

D (mm) = Fr / 3

or

Fr = D (mm) * 3

For example, if the French size is 9, the diameter is 9/3 = 3.0; mm.

From the basic math equation C = $π$ d, it follows that the catheter’s circumference in mm is only slightly (about 4.7%) greater than the French size.

An increasing French size corresponds to a larger external diameter. This is contrary to needle-gauge size, where an increasing gauge corresponds to a smaller diameter needle.

The French size is a measure of the external diameter of a catheter (not internal drainage channel). 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.

French catheter scale

French Gauge	Circumference (mm)	Diameter (mm)	Diameter (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	8.34	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