How to measure the size of a ring

Inside Circumference
Sizes
MM
Inches
Australian & British  
 US & Canada
French & Russian
Swiss
54.51
2.15
O
7
55 1/4
15 1/4
54.82
2.16
O 1/2
7 1/4
-
-
55.76
2.20
P
7 1/2
56 1/2
16 1/2
56.45
2.22
P 1/2
7 3/4
-
-
57.15
2.25
Q
8
57 3/4
17 3/4
57.65
2.27
Q 1/2
8 1/4
-
-
58.21
2.29
Q 3/4
8 1/2
-
-
58.47
2.30
R
8 5/8
59
19
58.72
2.31
R 1/4
8 3/4
-
-
59.06
2.32
R 1/2
8 7/8
-
-
59.34
2.35
R 3/4
9
-
-
60.00
2.36
S
9 1/8
60 1/4
20 1/4
60.38
2.38
S 1/4
9 1/4
-
-
60.66
2.39
S 1/2
9 3/8
-
-
60.98
2.40
S 3/4
9 1/2
-
-
61.29
2.41
T
9 5/8
61 1/2
21 1/2
61.64
2.43
T 1/4
9 3/4
-
-
62.33
2.45
T 1/2
10
-
-
62.89
2.48
U
10 1/4
62 3/4
22 3/4
63.46
2.50
U 1/2
10 1/2
-
-
63.84
2.51
V
10 5/8
63
23 3/4
64.21
2.53
V 1/4
10 3/4
-
-
64.97
2.56
V 1/2
11
-
-
65.22
2.57
W
11 1/8
65
25
65.50
2.58
W 1/4
11 1/4
-
-
65.78
2.59
W 1/2
11 3/8
-
-
66.22
2.61
W 3/4
11 1/2
-
-
66.54
2.62
X
11 5/8
66 1/4
26 1/4
66.73
2.63
X 1/4
11 3/4
-
-
66.92
2.64
X 1/2
11 7/8
-
-
67.51
2.66
Y
12
67 1/2
27 1/2
68.14
2.68
Y 1/2
12 1/4
-
-
68.77
2.71
Z
12 1/2
68 3/4
28 3/4
69.43
2.73
Z +1/2
12 3/4
-
-
70.15
2.76
Z+1
13
-
-
71.00
2.80
Z+1.5
13.5
-

This guide applies to all Viking rings