French catheter scale

The French scale or French gauge system is commonly used to measure the size (outside diameter) of a catheter. It is most often abbreviated as Fr, but can often abbreviated as FR or F. It may also be abbreviated as CH or Ch (for Charrière, its inventor) in French speaking countries. 1 Fr = 0.33 mm, and therefore the diameter of the catheter in millimeters 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 3 mm.

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

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 Gauge	Diameter (mm)	Diameter (inches)
3	1	0.039
4	1.35	0.053
5	1.67	0.066
6	2	0.079
7	2.3	0.092
8	2.7	0.105
9	3	0.118
10	3.3	0.131
11	3.7	0.144
12	4	0.158
13	4.3	0.170
14	4.7	0.184
15	5	0.197
16	5.3	0.210
17	5.7	0.223
18	6	0.236
19	6.3	0.249
20	6.7	0.263
22	7.3	0.288
24	8	0.315
26	8.7	0.341
28	9.3	0.367
30	10	0.393
32	10.7	0.419
34	11.3	0.445

External links

• Texloc