To determine your ring size:

1) Use a piece of string or dental floss and wrap it around the base of your finger.
2) Use a pen to mark the point on the string where the end meets.
3) Use a ruler to measure the string.
4) Choose the closest measurement to the chart below to find your ring size.

Alternatively, you can download this handly sizing tool.

Inside Diameter  Inside Circumference Sizes
MM Inches MM Inches British & Australian  US & Canada
15.70 0.618 49.32 1.94 J 1/2 5
16.10 0.634 50.58 1.99 L 5.5
16.51 0.650 51.87 2.04 M 6
16.92 0.666 53.16 2.09 N 6.5
17.35 0.683 54.51 2.15 O 7
17.75 0.699 55.76 2.20 P 7.5
18.19 0.716 57.15 2.25 Q 8
18.53 0.729 58.21 2.29 Q 3/4 8.5
18.89 0.748 59.34 2.35 R 3/4 9
19.41 0.764 60.98 2.40 S 3/4 9.5
19.84 0.781 62.33 2.45 T 1/2 10
20.20 0.797 63.46 2.50 U 1/2 10.5
20.68 0.814 64.97 2.56 V 1/2 11
21.08 0.830 66.22 2.61 W 3/4 11.5
21.49 0.846 67.51 2.66 Y 12
21.89 0.862 68.77 2.71 Z 12.5
22.33 0.879 70.15 2.76 Z+1 13
22.60 0.890 71.00 2.80 Z+1.5 13.5