The most common ring size in America is different for men and women. Men's average ring size is from 10 to 11 while women's are from 6 to 7. Most jewlers have a ring chart for you to find the right size.
In the United Kingdom, ring sizes are determined using an alphabetic system, from letter A to Z. Other European countries use a number system like the United States, but they use millimetres from 38 to 70.
The ring size can change at any time due to weight loss or if you're still growing. Make sure to get a proper measurement before purchasing your ring.
Rings have been used as promise or wedding rings since as far back as 1477. Men's wedding rings did not become popular until World War II. They were mementos for the soldiers.