What is my ring size?

Put the ring that fits you the most on top of a ruler or a meter-measure and measure its inside diameter from side to side of the inside circle. For example:

15 mm diameter (one centimeter and five mm) - the size of your ring is 5
16 mm diameter (one centimeter and
six mm) - the size of your ring is 6
17 mm diameter (one centimeter and seven mm) - the size of your ring is 7
18 mm diameter (one centimeter and 8 mm) - the size of your ring is 8

Or if you know your size so 

50=5

52=6

54=7