French catheter scale chart 

The French catheter scale or "French units" (Fr) is commonly used to measure the outside diameter of needles, catheters, and other cylindrical medical instruments.

1 Fr is equivalent to 0.33 mm = .013" = 1/77" of diameter. Thus, the size in French units is roughly equal to the circumference of the catheter in millimetres.

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

References

This chart taken from Texloc

See also