The best way to measure your ring size is to visit your local jewelry store. If you are unable to do that, you can measure your finger with a ribbon, then measure the distance using a ruler. 

Ring Sizing Chart:

INNER CIRCUMFERENCE (MM) DIAMETER (MM) RING SIZE
44.2 14.1 3
44.8 14.3 3.25
45.5 14.5 3.5
46.1 14.7 3.75
46.8 14.9 4
47.4 15.1 4.25
48 15.3 4.5
48.7 15.5 4.75
49.3 15.7 5
50 15.9 5.25
50.6 16.1 5.5
51.2 16.3 5.75
51.9 16.5 6
52.5 16.7 6.25
53.1 16.9 6.5
53.8 17.1 6.75
54.4 17.3 7
55.1 17.5 7.25
55.7 17.7 7.5
56.3 17.9 7.75
57 18.1 8
57.6 18.3 8.25
58.3 18.5 8.5
58.9 18.8 8.75
59.5 19 9
60.2 19.2 9.25
60.8 19.4 9.5
61.4 19.6 9.75
62.1 19.8 10
62.7 20 10.25
63.4 20.2 10.5
64 20.4 10.75
64.6 20.6 11