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 = πd, 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 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 |
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