The best way to figure it out is visiting local jeweller or any jewelry store. Professional will help you better than any home method! Here at Stellar Adron we use US size standards and you can use this trustworthy conversion chart in case if you got different standards measurements.
Internal Diameter (mm)
Circumference (mm)
US
UK/AU
EU
14.1
44.2
3
F
44
14.5
45.5
3.5
G
45
14.9
46.7
4
H
46
15.3
48
4.5
I
47
15.8
49.3
5
J
49
16.1
50.6
5.5
K
50
16.6
51.9
6
L
52
16.9
53.1
6.5
M
53
17.3
54.4
7
N
54
17.7
55.8
7.5
O
56
18
57.2
8
P
57
18.5
58.2
8.5
Q
58
19
59.5
9
R
60
19.4
60.9
9.5
S
61
19.8
62.1
10
T
62
20.2
63.3
10.5
U
63
20.6
64.9
11
V
65
21
66.2
11.5
W
66
21.4
67.2
12
X
67
21.8
68.4
12.5
Y
68
22.2
70.2
13
Z
70
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