The British ring size system differs from those used in the US and Asia in that the sizes go from A-Z rather than being numeric values. The system also uses a number of 'oversizes' denoted as Z+1, Z+2 etc and half sizes A½, B½ etc. The system dates from early Victorian times when jewellers started producing stock sized rings.

The last British standard BS6820, issued in 1987, defines the ring sizes by inner circumference in mm. Each full size change being an increase of 1.25 mm in inner circumference, thus each half size is an increase of 0.625mm in inner circumference. Technically BS6820 has been superseded by BS28653 defining ring size by inner circumference but British ring sizes A-Z are still in general use.

British ring size dimensions in inches & millimetres

Size Inner Diameter Inner Circumference   in mm in mm A 0.470 11.94 1.476 37.500 A½ 0.478 12.14 1.501 38.125 B 0.487 12.33 1.526 38.750 B½ 0.493 12.53 1.550 39.375 C 0.501 12.73 1.575 40.000 C½ 0.509 12.93 1.599 40.625 D 0.517 13.13 1.624 41.250 D½ 0.525 13.33 1.649 41.875 E 0.533 13.53 1.673 42.500 E½ 0.540 13.73 1.698 43.125 F 0.548 13.93 1.722 43.750 F½ 0.556 14.13 1.747 44.375 G 0.564 14.32 1.772 45.000 G½ 0.572 14.52 1.796 45.625 H 0.580 14.72 1.821 46.250 H½ 0.587 14.92 1.845 46.875 I 0.595 15.12 1.870 47.500 I½ 0.603 15.32 1.895 48.125 J 0.611 15.52 1.919 48.750 J½ 0.619 15.72 1.944 49.375 K 0.627 15.92 1.969 50.000 K½ 0.634 16.11 1.993 50.625 L 0.642 16.31 2.018 51.250 L½ 0.650 16.51 2.042 51.875 M 0.658 16.71 2.067 52.500 M½ 0.669 16.91 2.092 53.125 N 0.674 17.11 2.116 53.750 N½ 0.681 17.31 2.141 54.375 O 0.689 17.51 2.165 55.000 O½ 0.697 17.71 2.190 55.625

Size Inner Diameter Inner Circumference   in mm in mm P 0.705 17.90 2.215 56.250 P½ 0.713 18.10 2.239 56.875 Q 0.721 18.30 2.264 57.500 Q½ 0.728 18.50 2.288 58.125 R 0.736 18.70 2.313 58.750 R½ 0.744 18.90 2.338 59.375 S 0.752 19.10 2.362 60.000 S½ 0.760 19.30 2.387 60.625 T 0.768 19.50 2.411 61.250 T½ 0.775 19.70 2.436 61.875 U 0.783 19.89 2.461 62.500 U½ 0.791 20.09 2.485 63.125 V 0.799 20.29 2.510 63.750 V½ 0.807 20.49 2.534 64.375 W 0.815 20.69 2.559 65.000 W½ 0.822 20.89 2.584 65.625 X 0.830 21.09 2.608 66.250 X½ 0.838 21.29 2.633 66.875 Y 0.846 21.49 2.657 67.500 Y½ 0.854 21.68 2.682 68.125 Z 0.862 21.88 2.707 68.750 Z½ 0.869 22.08 2.731 69.375 Z+1 0.877 22.28 2.756 70.000 Z+2 0.893 22.68 2.805 71.250 Z+3 0.909 23.08 2.854 72.500 Z+4 0.924 23.47 2.904 73.750 Z+5 0.940 23.87 2.953 75.000 Z+6 0.956 24.27 3.002 76.250 Z+7 0.971 24.67 3.051 77.500 Z+8 0.987 25.07 3.100 78.750