This is a chart that will be extremely helpful for you to determine the ring size that you need. You can either make use of a tape or a small piece of string to measure the area that will be occupied by the ring. When it becomes a complete circle, make a mark on such string. This will be helpful for you to compare with the chart that is mentioned below. 

Inside Diameter

Inside Circumference

SIZE

MM

MM

US / Canada

UK / Australia / South Africa

France / Germany

Italy

Spain

14.1

44.2

3

F

44

4

4.5

14.5

45.5

3.5

G

46

5

5.5

14.9

46.8

4

H

47

6

7

15.3

48

4.5

I

48

8

8

15.7

49.3

5

J-½

50

10

9.5

16.1

50.6

5.5

K-½

51

11

10.5

16.5

51.9

6

L-½

52

12

12

16.9

53.1

6.5

M-½

53

13

13

17.3

54.4

7

N-½

54

15

14.5

17.7

55.7

7.5

O-½

56

16

16

18.1

57

8

P-½

57

17

17

18.5

58.3

8.5

Q-½

58

18

18.5

19

59.5

9

R-½

59

20

19.5

19.4

60.8

9.5

S-½

61

21

21

19.8

62.1

10

T-½

62

22

22

20.2

63.4

10.5

U-½

63

24

23.5

20.6

64.6

11

V-½

64

25

24.5

21

65.9

11.5

W-½

66

26

26

21.4

67.2

12

X-½

67

27

27.5

21.8

68.5

12.5

Z

68

28

28.5

22.2

69.7

13

Z+1

70

30

30

22.6

71

13.5

Z+1.5

71

31

31

23

72.3

14

Z+3

72

32

32.5

23.4

73.5

14.5

Z+4

73

33

33.5

23.8

74.8

15

Z+5

74

35

35