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The table below will be of some use to those interested properly describing the specification of their rings.

Jewelry Ring Size Comparison Chart (Millimetres)

US/CANADIAN
STANDARD

INSIDE
DIAMETER
Millimetres

 INSIDE
CIRCUMFERENCE
Millimetres
1 1/2 12.78 40.2
2 13.21 41.5
2-1/2 13.61 42.7
3 14.05 44.0
3-1/2 14.45 45.2
4 14.86 46.5
4-1/2 15.27 47.8
5 15.70 49.0
5-1/2 16.10 50.2
6 16.51 51.5
6-1/2 16.92 52.8
7 17.35 54.0
7-1/2 17.35 55.3
8 17.63 56.6
8-1/2 18.19 57.8
9 18.59 59.1
9-1/2 18.99 60.3
10 19.41 61.6
10-1/2 19.84 62.8
11 20.24 64.1
11-1/2 20.68 66.0
12 21.01 67.3

 

Jewelry Ring Size Comparison Chart
(Inches and Millimetres)

US/CANADIAN
STANDARD

INSIDE
DIAMETER
MM/INCHES

 INSIDE
CIRCUMFERENCE
MM/INCHES
1 1/2 12.78 / .503 40.2 / 1.580
2 13.21/.520 41.5/1.634
2-1/2 13.61/.536 42.7/1.684
3 14.05/.553 44.0/1.739
3-1/2 14.45/.569 45.2/1.788
4 14.86/.585 46.5/1.836
4-1/2 15.27/.601 47.8/1.888
5 15.70/.618 49.0/1.941
5-1/2 16.10/.634 50.2/1.992
6 16.51/.650 51.5/2.042
6-1/2 16.92/.666 52.8/2.092
7 17.35/.683 54.0/2.146
7-1/2 17.35/.683 55.3/2.196
8 17.63/.699 56.6/2.249
8-1/2 18.19/.716 57.8/2.300
9 18.59/.732 59.1/2.350
9-1/2 18.99/.748 60.3/2.400
10 19.41/.764 61.6/2.463
10-1/2 19.84/.781 62.8/2.504
11 20.24/.797 64.1/2.567
11-1/2 20.68/.814 66.0/2.608
12 21.01/8.30 67.3/2.658

 

ISO Ring Size 
Inside diameter  Inside circumference  USA
Canada 		 UK/Ireland
Australia		 Japan  Switzerland
(inches)  (millimetres)  (inches)  (millimetres) 
0.458 11.633 1.438 36.528 0         
0.466 11.836 1.463 37.166  1/4      
0.474 12.040 1.488 37.804  1/2 A     
0.482 12.243 1.513 38.442  3/4 A 1/2     
0.49 12.446 1.539 39.080 1    B  1  
0.498 12.649 1.564 39.718 1 1/4 B 1/2     
0.506 12.852 1.589 40.357 1 1/2 C     
0.514 13.056 1.614 40.995 1 3/4 C 1/2     
0.522 13.259 1.639 41.633 2    D  2 1.5
0.53 13.462 1.664 42.271 2 1/4 D 1/2     
0.538 13.665 1.689 42.909 2 1/2 E  3 2.75
0.546 13.868 1.714 43.547 2 3/4 E 1/2     
0.554 14.072 1.740 44.185 3    F  4 4
0.562 14.275 1.765 44.823 3 1/4 F 1/2  5 5.25
0.57 14.478 1.790 45.461 3 1/2 G     
0.578 14.681 1.815 46.099 3 3/4 G 1/2  6 6.5
0.586 14.884 1.840 46.737 4    H  7  
0.594 15.088 1.865 47.375 4 1/4 H 1/2    7.75
0.602 15.291 1.890 48.013 4 1/2 I  8  
0.61 15.494 1.915 48.651 4 3/4 J    9
0.618 15.697 1.941 49.289 5    J 1/2  9  
0.626 15.900 1.966 49.927 5 1/4 K    10
0.634 16.104 1.991 50.565 5 1/2 K 1/2  10  
0.642 16.307 2.016 51.203 5 3/4 L    11.75
0.65 16.510 2.041 51.841 6    L 1/2  11 12.75
0.658 16.713 2.066 52.479 6 1/4 M  12  
0.666 16.916 2.091 53.117 6 1/2 M 1/2  13 14
0.674 17.120 2.116 53.756 6 3/4 N     
0.682 17.323 2.141 54.394 7    N 1/2  14 15.25
0.69 17.526 2.167 55.032 7 1/4 O     
0.698 17.729 2.192 55.670 7 1/2 O 1/2  15 16.5
0.706 17.932 2.217 56.308 7 3/4 P     
0.714 18.136 2.242 56.946 8    P 1/2  16 17.75
0.722 18.339 2.267 57.584 8 1/4 Q     
0.73 18.542 2.292 58.222 8 1/2 Q 1/2  17  
0.738 18.745 2.317 58.860 8 3/4 R    19
0.746 18.948 2.342 59.498 9    R 1/2  18  
0.754 19.152 2.368 60.136 9 1/4 S    20.25
0.762 19.355 2.393 60.774 9 1/2 S 1/2  19  
0.77 19.558 2.418 61.412 9 3/4 T    21.5
0.778 19.761 2.443 62.050 10    T 1/2  20  
0.786 19.964 2.468 62.688 10 1/4 U  21  
0.794 20.168 2.493 63.326 10 1/2 U 1/2  22 22.75
0.802 20.371 2.518 63.964 10 3/4 V     
0.81 20.574 2.543 64.602 11    V 1/2  23  
0.818 20.777 2.569 65.240 11 1/4 W    25
0.826 20.980 2.594 65.878 11 1/2 W 1/2  24  
0.834 21.184 2.619 66.517 11 3/4 X     
0.842 21.387 2.644 67.155 12    X 1/2  25 27.5
0.85 21.590 2.669 67.793 12 1/4 Y     
0.858 21.793 2.694 68.431 12 1/2 Z  26 28.75
0.866 21.996 2.719 69.069 12 3/4 Z 1/2     
0.874 22.200 2.744 69.707 13