You can measure your ring size with a ruler.

1. Place a well fitting ring from yourself or from the person for whom you want to buy a ring on the ruler.
2. See how many mm the ruler indicates on the inside of your ring. For example, if this is 18 mm, your ring size is 18.

RING SIZES	DIAMETER IN MM	CIRCUMFERENCE IN MM
14	14	44
15	15	47,1
16	16	50,3
17	17	53,4
18	18	56,5
19	19	59,7
20	20	62,8
21	21	66
22	22	69,1