Two ways, same result.

A. Mesure your ring(from the inside)
1. Select an existing ring that fits properly
2. Mesure with a straight line the internal diameter of the ring
3. Use the chart below to determine the size ring.  

Internal diameter (mm)

15,6 - 15,9

16,5

16,9 - 17,9

17,2 - 17,83

17,9 - 18,5

Internal diameter (inches)

0,61 - 0,63

0,65 - 0,66

0,67 - 0,68

0,69 - 0,7

0,71 - 0,73

SP/FR

10

12

14

16

18

EU

50

52

54

56

56

UK

J-K

L-L 1/2

M 1/2 - N 1/2

O - P

P 1/2- Q 1/2

US/CANADA

5 1/2

6-6 1/2

7

7 1/2 - 8

8 1/2 - 9

B. Measure your finger
1. Wrap a strip of paper around your finger where you'd like your ring to be.
2. Make sure that the paper is pulled snug to your finger, the tighter the better, to find your best fit.
3. Mark the spot where the paper meets and measure the distance with a ruler(mm)
4. Use the chart above to determine your ring size.