Catheter Sizes	Diameter inches	Diameter MM
3	.039	1
4	.053	1.35
5	.066	1.67
6	.079	2.0
7	.92	2.3
8	.105	2.7
9	.118	3.0
10	.131	3.3
11	.144	3.7
12	.158	4.0
13	.170	4.3
14	.184	4.7
15	.197	5.0
16	.210	5.3
17	.223	5.7
18	.236	6.0
19	.249	6.3
20	.263	6.7
22	.288	7.3
24	.315	8.0
26	.341	8.7
28	.367	9.3
30	.393	10.0
32	.419	10.7
34	.445	11.3

• Diameter in MM = Catheter Size/3
• Diameter in Inches = Catheter Size/76.2
• For oval or other shapes, measure the circumference in MM using a paper strip then multiply this length by 0.955 to obtain French size.