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 centimeters and
six mm) - the size of your ring ia 6
17 mm diameter (one centimeters and seven mm) - the siE of your ring is 7
18 mm diameter (one centimeters and 8 mm) - the size of your rung is 8

 

Or if you know your size so 

50=5

52=6

54=7