Circumference

• 1). Wrap a piece of string around the base of your ring finger. Mark the string where the end meets it.
  
  
• 2). Unwrap the string and lay it on a ruler that measures millimeters. Measure to the marked area.
  
  
• 3). Consult a ring sizing chart (see Resources) to find your ring size. For example, if the circumference of your ring finger measured 46.7 millimeters, your ring size would be a U.S. size 4.
  
  

Diameter

• 1). Choose a ring from your personal collection that fits you comfortably.
  
  
• 2). Measure across the inside of the ring with a ruler that measures millimeters.
  
  
• 3). Consult a ring sizing chart (see Resources) to find your ring size. For example, if the diameter of your ring measured 14.9 millimeters, your ring size would be a U.S. size 4.