The safest way to find out the exact size of your finger is in the presence of a jeweler. If you can’t do this, there are many other ways you can approximate it.

Find a thin strip of paper or piece of string. Wrap it around the finger. With a pen, mark the point on the paper or string where it overlaps. Using a flat ruler, measure the length of the paper string from the starting point to the pen mark.

You cand also use this metod with the inner circumference (French Sizes scale), if you have an old ring.


Use the international ring size conversion table to find the matching size: 
  Interior
Diameter (mm)		 Inside
Circumference (mm)		 USA
Canada
Mexico		 UK
Australia
Ireland		 Italy
Spain
Switzerland		 China Japan
South America
             
1 13.06 41 1¾ C½ 1  
2 13.26 42 2 D 1.75 2
3 13.67 43 2½ E 3 3
4 13.87 44 2¾ E½ 3.5  
5 14.27 45 3¼ F½ 4.75 5
6 14.68 46 3¾ G½ 6 6
7 15.09 47 4¼ H½ 7.5  
8 15.29 48 4½ I 8 8
9 15.49 49 4¾ J 8.75  
10 15.9 50 5¼ K 10  
11 16.31 51 5¾ L 11.25  
12 16.51 52 6 L½ 11.75 11
13 16.92 53 6½ M½ 13.25 13
14 17.32 54 7 N½ 14.5 14
15 17.53 55 7¼ O 15  
16 17.93 56 7¾ P 16.25  
17 18.14 57 8 P½ 17 16
18 18.54 58 8½ Q½ 18.25 17
19 18.75 59 8¾ R 19  
20 19.15 60 9¼ S 20.25  
21 19.35 61 9½ S½ 20.75 19
22 19.76 62 10 T½ 22 20
23 19.96 63 10¼ U 22.75 21
24 20.37 64 10¾ V 24  
25 20.57 65 11 V½ 24.75 23
26 20.98 66 11½ W½ 26 24
27 21.39 67 12 X½ 27.25 25
28 21.59 68 12¼ Y 27.75  
29 22 69 12¾ Z½ 29  
30 22.2 70 13   29.75 27
31 22.61 71 13½   31  
32 22.81 72 13¾ Z2 31.75  
33 23.22 73 14¼   33  
34 23.62 74 14¾   34.25  
35 23.83 75 15   34.75  
36 24.23 76 15½   36.25  

Measure your finger at the end of the day.
Your finger size changes depending on the weather and time of day. For best results, measure your finger size at the end of the day when your fingers are warm (early in the morning, your fingers are cold and therefore smaller).