If you want to know what ring size you need to fit your finger without going to a jeweler here is a quick and easy chart to size your finger.

Ring Size    Circumference (in mm)    Diameter (in mm)
5                  53 mm                               17 mm
6                  55 mm                               17.5 mm
7                  57 mm                               18 mm 
8                  60 mm                               19 mm
9                  63 mm                               20 mm
10                66 mm                               21 mm
11                69 mm                               22 mm
12                72 mm                               23 mm