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.

 
Diameter in USA Size (Our measure) UK Size Circumference (DE, FR Size)
15 mm 4 H 46.8
15.3 mm 4.5 I 48
15.6 mm 5 J 49.3
16.2 mm 5.5 K 50.6
16.6 mm 6 L 51.9
16.9 mm 6.5 M 53.1
17.2 mm 7 N 54.4
17.8 mm 7.5 O 55.7
18.1 mm 8 P 57
18.5 mm 8.5 Q 58.3
19.1 mm 9 R 59.5
19.4 mm 9.5 S 60.8
19.7 mm 10 T 62.1
20.4 mm 10.5 U 63.4
20.7 mm 11 V 64.6
21.0 mm 11.5 W 65.9
21.6 mm 12 X 67.2
22.0 mm 12.5 Y 68.5
22.3 mm 13 Z 69.7
22.9 mm 13.5 Z+2 71
23.2 mm 14 Z+3 72.3
23.6 mm 14.5 Z+4 73.6
24.0 mm 15 Z+5 74.8
Size Chart
Size Chart