There are several systems for denoting the sizes of jewellery rings in use around the world:

 

• The ISO standard for ring sizes is ISO 8653:1986, which defines standard ring sizes in terms of the inner circumference of the ring measured in millimeters.

• In the United States and Canada, ring sizes are specified using a numerical scale, with quarter and half sizes. An increase of a full size is an increase of 0.032 inch (0.8128 mm) in diameter, or roughly 1/10 inch (more precisely, 0.1005 in or 2.55 mm) in inside circumference.

• In Ireland, the United Kingdom and Australia,[2] ring sizes are specified using an alphabetical scale, with half sizes.

• In India, Japan and China, ring sizes are specified using a numerical scale, that only has whole sizes, and does not have simple linear correlation with diameter or circumference.

• In Austria, France, Germany, Belgium and Scandinavia, ring sizes are specified using actual internal circumference in mm (the same as the ISO standard). In some countries, half sizes may be used.

• In Italy, Spain, Netherlands, and Switzerland, ring sizes are specified as the circumference minus 40mm. A 50 in Germany will be a 10 in Switzerland.
  
  While I always recommend that you get properly sized at a local jewelry store however, there is a simple process that you can use at home.
   

• Finding your ring is a very simple process that can be done with minimal resources. All you need is a ruler and a piece of string.
   

• 1. Wrap the string around your finger and mark where it begins to overlap. 

• 2. Straighten the string out and measure it with a ruler. This is the circumference measurement of your finger. 

• 3. Compare this with the size chart below.