This converter requires the use of Javascript enabled and capable browsers. This script converts a selected ring size from any of the possible regions, to the ring size designation used in other countries. Select any ONE ring size in the drop box selectors, any that are directly convertible will be shown in the region applicable. This is a very close approximation.

You can read more detailed article here : How to choose the correct ring size
 

Regional Standards	Size Designation
Select Any Single Ring Size Conversion
American	0000 00 0 1/2 3/4 1 1 1/4 1 1/2 1 3/4 2 2 1/4 2 1/2 2 3/4 3 3 1/8 3 1/4 3 3/8 3 1/2 3 5/8 3 3/4 4 4 1/4 4 1/2 4 5/8 4 3/4 5 5 1/8 5 1/4 5 3/8 5 1/2 5 3/4 5 7/8 6 6 1/4 6 1/2 6 3/4 7 7 1/4 7 1/2 7 3/4 8 8 1/4 8 1/2 8 5/8 8 3/4 8 7/8 9 9 1/8 9 1/4 9 3/8 9 1/2 9 5/8 9 3/4 10 10 1/4 10 1/2 10 5/8 10 3/4 11 11 1/8 11 1/4 11 3/8 11 1/2 11 5/8 11 3/4 11 7/8 12 12 1/2 12 3/4 13 ********** Size
British	---   ---   --- A A 1/2 B B 1/2 C C 1/2 D D 1/2 E E 1/2 F F 1/2   --- G   --- G 1/2 H H 1/2 I I 1/2 J   --- J 1/2 K   --- K 1/2 L   --- L 1/2 M M 1/2 N N 1/2 O O 1/2 P P 1/2 Q Q 1/2   --- R   --- R 1/2   --- S   --- S 1/2   --- T   --- T 1/2 U U 1/2 V   --- V 1/2 W   --- W 1/2   --- X   --- X 1/2 Y Z Z 1/2   --- ********** Size
French	---   ---   ---   ---   ---   ---   ---   ---   --- 41 1/2   --- 42 3/4   --- 44   ---   --- 45 1/4   ---   --- 46 1/2   --- 47 3/4   --- 49   ---   --- 50   ---   --- 51 3/4   ---   --- 52 3/4   --- 54   --- 55 1/4   --- 56 1/2   --- 57 3/4   ---   --- 59   ---   ---   --- 60 1/4   ---   ---   --- 61 1/2   ---   --- 62 3/4   --- 63   ---   --- 65   ---   ---   --- 66 1/4   ---   --- 67 1/2 68 3/4   ---   --- ********** Size
German	---   ---   ---   ---   ---   ---   ---   ---   --- 13 1/2   --- 13 3/4   --- 14   ---   ---   --- 14 1/2   ---   --- 15   --- 15 1/4 15 1/2   --- 15 3/4   ---   ---   --- 16   ---   --- 16 1/2   --- 17   --- 17 1/4   --- 17 3/4   --- 18   --- 18 1/2   ---   ---   --- 19   ---   ---   --- 19 1/2   ---   --- 20   --- 20 1/4   ---   --- 20 3/4   ---   ---   --- 21   ---   ---   --- 21 1/4 21 3/4   --- 22 ********** Size
Japanese	---   ---   ---   ---   --- 1   ---   ---   --- 2   --- 3   --- 4   ---   --- 5   --- 6   --- 7   --- 8   ---   --- 9   ---   --- 10   ---   ---   --- 12   --- 13   --- 14   --- 15   --- 16   --- 17   ---   ---   --- 18   ---   ---   --- 19   ---   --- 20 21 22   ---   --- 23   ---   ---   --- 24   ---   ---   --- 25 26   --- 27 ********** Size
Swiss	---   ---   ---   ---   ---   ---   ---   ---   --- 1 1/2   --- 2 3/4   --- 4   ---   --- 5 1/4   ---   --- 6 1/2   --- 7 3/4   --- 9   ---   --- 10   ---   --- 11 3/4   ---   --- 12 3/4   --- 14   --- 15 1/4   --- 16 1/2   --- 17 3/4   ---   --- 19   ---   ---   --- 20 1/4   ---   ---   --- 21 1/2   ---   --- 22 3/4   --- 23 3/4   ---   --- 25   ---   ---   --- 26 1/4   ---   --- 27 1/2 28 3/4   ---   --- ********** Size
Circumference	---   ---   ---   ---   --- 38.9 39.6 40.2 40.8 41.5 42.1 42.7 43.4 44.0 44.3 44.6 44.9 45.2 45.6 45.9 46.5 47.1 47.8 48.1 48.4 49.0 49.3 49.6 50.0 50.3 50.9 51.3 51.5 52.2 52.8 53.4 54.0 54.7 55.3 55.9 56.6 57.2 57.8 58.4 58.4 59.0 59.1 59.4 59.7 60.0 60.3 60.6 60.9 61.6 62.2 62.8 63.1 63.5 64.1 64.3 64.7 65.0 65.3 65.7 66.0 66.3 66.6 67.9 68.5 69.1 ********** mm
Diameter	.390 .422 .454 .474 .482 .487   --- .503   --- .520   --- .536   --- .553   ---   ---   --- .569   ---   --- .585   --- .601   ---   --- .618   ---   ---   --- .634   ---   --- .650   --- .666   --- .683   --- .699   --- .716   --- .732   ---   ---   --- .748   ---   ---   --- .764   ---   --- .781   --- .797   ---   --- .814   ---   ---   ---   --- .830   ---   --- .846 .862   --- .879 ********** inches
Diameter	9.91 10.72 11.53 11.95 12.18 12.37 12.60 12.78 13.00 13.21 13.41 13.61 13.83 14.05 14.15 14.25 14.36 14.45 14.56 14.65 14.86 15.04 15.27 15.40 15.53 15.70 15.80 15.90 16.0 16.10 16.30 16.41 16.51 16.71 16.92 17.13 17.35 17.45 17.75 17.97 18.19 18.35 18.53 18.61 18.69 18.80 18.89 19.10 19.22 19.31 19.41 19.51 19.62 19.84 20.02 20.20 20.32 20.44 20.68 20.76 20.85 20.94 21.08 21.18 21.24 21.30 21.49 21.89 22.10 22.33 ********** mm