This chart will help you determine the ring size you will require.  Use a tape measure, piece of string or piece of string around your finger, where you will be wearing the ring.  Mark the string or paper where it overlaps to form a full circle.  Then take the marked string or paper and measure it so you can compare it with the below chart.  If it is a wide band, you might wish to add one size to accommodate for the extra width. 

Ring Size    Inches           mm
4                 1-13/16         46.5
4 ½             1-7/8             47.8
5                 1-15/16         49
5 ½              2                  50.3
6                  2-1/16          51.5
6 ½              2-1/8            52.8
7                  2-3/16          54
7 ½              2-1/4            55.3
8                  2-5/16          56.6
8 ½              2-3/8            57.6
9                  2-7/16          59.1
9 ½              2-1/2            60.3
10                2-9/16          61.6
10 ½            2-5/8            62.8
11                2-11/16        64.1
11 ½            2-3/4            65.3
12                2-13/16        66.6
12 ½            2-7/8            67.9
13                2-15/16        69.1

Below is another chart that will help you to determine your size using the inside diameter of the ring in millimeters.  This is often a way to double check your findings from the paper method.  You can also use a ring you currently wear and cross check it with the below chart.

Ring Size       Inside Diameter of Ring in millimeters(mm)
3                     14
4                     15
5                     15.5
6                     16
7                     17
8                     18
9                     19
10                   20
11                   20.5
12                   21.5
13                   22.5
14                   23.5
  

Whenever there is any doubt you can always use a professional sizing stick or visit a local jeweler and they will often size your finger for free.