Place one of your rings on a ruler or tape measure and measure the inside diameter of the ring from side to side of the inner circle.
Watch the sample video:




Diameter 12 mm (one centimeter and two millimeters) - your ring size 2.5
Diameter 13 mm (one centimeter and three millimeters) - your ring size 3.5

Diameter 15 mm (one centimeter and five millimeters) - your ring size is 5
Diameter 16 mm (one centimeter and six millimeters) - your ring size is 6
Diameter 17 mm (one centimeter and seven millimeters) - your ring size is 7
Diameter 18 mm (one centimeter and eight millimeters) - your ring size is 8