When choosing the right ring size, you should know that sizes are always measured by mm, but they're indexed differently around the globe.
also keep in mind that sizes may differ between your left & right hand!

 Ring Size Chart (by M"M)

EU	U.S	DIAMETER	FINGER
46	3.75	14.65	46.1
47	4	14.97	47
48	4.5	15.3	48
49	5	15.6	49
50	5.25	15.9	50
51	5.50	16.24	51
52	6	16.5	52.1
53	6.25	16.8	52.8
54	7	17.2	54
55	7.25	17.5	55.1
56	7.50	17.8	56
57	8	18.1	57
58	8.25	18.4	58
59	8.50	18.8	58.9
60	9	19.1	60
61	9.25	19.4	61
62	9.50	19.7	62.1
63	10.25	20.0	63
64	10.5	20.3	64
65	11	20.7	65
66	11.5	21.02	65.9
67	12	21.34	66.9
68	12.25	21.66	68
69	12.5	21.97	69.1
70	13	22.29	70