Diameter (mm) Circumference (mm) US/Canada GB / IRL / AUS Japanese Swiss
           
11.6 36.5 0      
11.8 37.2 1/4      
12.0 37.8 1/2 A    
12.2 38.4 3/4 A 1/2    
12.4 39.1 1 B 1  
12.6 39.7 1 1/4 B 1/2    
12.9 40.4 1 1/2 C    
13.1 41.0 1 3/4 C 1/2    
13.3 41.6 2 D 2 1.50
13.5 42.3 2 1/4 D 1/2    
13.7 42.9 2 1/2 E 3 2.75
13.9 43.5 2 3/4 E 1/2    
14.1 44.2 3 F 4 4.00
14.3 44.8 3 1/4 F 1/2 5 5.25
14.5 45.5 3 1/2 G    
14.7 46.1 3 3/4 G 1/2 6 6.50
14.9 46.7 4 H 7  
15.1 47.4 4 1/4 H 1/2   7.75
15.3 48.0 4 1/2 I 8  
15.5 48.7 4 3/4 J   9.00
15.7 49.3 5 J 1/2 9  
15.9 49.9 5 1/4 K   10.00
16.1 50.6 5 1/2 K 1/2 10  
16.3 51.2 5 3/4 L   11.75
16.5 51.8 6 L 1/2 11 12.75
16.7 52.5 6 1/4 M 12  
16.9 53.1 6 1/2 M 1/2 13 14.00
17.1 53.8 6 3/4 N    
17.3 54.4 7 N 1/2 14 15.25
17.5 55.0 7 1/4 O    
17.7 55.7 7 1/2 O 1/2 15 16.50
17.9 56.3 7 3/4 P    
18.1 56.9 8 P 1/2 16 17.75
18.3 57.6 8 1/4 Q    
18.5 58.2 8 1/2 Q 1/2 17  
18.7 58.9 8 3/4 R   19
18.9 59.5 9 R 1/2 18  
19.2 60.1 9 1/4 S   20.25
19.4 60.8 9 1/2 S 1/2 19  
19.6 61.4 9 3/4 T   21.5
19.8 62.1 10 T 1/2 20  
20.0 62.7 10 1/4 U 21  
20.2 63.3 10 1/2 U 1/2 22 22.75
20.4 64.0 10 3/4 V    
20.6 64.6 11 V 1/2 23  
20.8 65.2 11 1/4 W   25
21.0 65.9 11 1/2 W 1/2 24  
21.2 66.5 11 3/4 X    
21.4 67.2 12 X 1/2 25 27.50
21.6 67.8 12 1/4 Y    
21.8 68.4 12 1/2 Z 26 28.75
22.0 69.1 12 3/4 Z 1/2    
22.2 69.7 13   27  
22.4 70.3 13 1/4 Z1    
22.6 71.0 13 1/2      
22.8 71.6 13 3/4 Z2    
23.0 72.3 14 Z3    
23.2 72.9 14 1/4      
23.4 73.5 14 1/2 Z4    
Although this chart won't figure out your loved ones ring size without them knowing, it will help you convert from one ring size to another. You can work out your own ring size by downloading the Nuttalls of Chester Ring Size Finder and following the instructions.

Find out her ring size Secretly: Remember, a woman's ring size is different for each finger. If she leaves her rings lying around, make an impression in a bar of soap or trace the inside of the ring on a piece of paper. You might try one of her rings on your own finger. Push it down as far as you can, mark the spot with a pen, then go straight to the jeweler and let him measure it for you. Alternatively, her friends may be able to help you out by asking her for her ring size or trying on one of her rings to compare it to their own. Best of all, slip one of her rings into your pocket and show it to a jeweler…if you can do it without arousing suspicion. Another idea is ask her parents for help, that is if you have a good relationship with them!

Best of luck....!