Ring Sizes How we measure ring sizes All our rings are measured and standardised with a new Wheatsheaf measuring stick. The reading is taken as centre, with a tolerance of one quarter of a size. If you have your finger measured by a Jeweller, always check if the size is centre, leading edge or trailing edge. There is no substitute for having your finger measured professionally, but it's not an exact science. Our fingers change in size from morning to evening, month to month and even Winter to Summer. The following chart is helpful to see how the sizes compare. UK: Diameter mm Circumference mm American A 11.5 36.25 0.5 B 12.3 38.64 1 C 12.7 39.90 1.5 D 13.1 41.15 2 E 13.5 42.41 2.5 F 13.9 43.67 3 G 14.3 44.92 3.25 H 14.7 46.18 3.75 I 15.1 47.44 4.25 J 15.5 48.69 4.75 K 15.9 49.95 5.25 L 16.3 51.24 5.75 M 16.7 52.46 6.25 N 17.1 53.72 6.75 O 17.5 54.98 7 P 17.9 56.23 7.5 Q 18.3 57.49 8 R 18.7 58.75 8.5 S 19.1 60.00 9 T 19.5 61.26 9.5 U 19.9 62.52 10 V 20.3 63.77 10.5 W 20.7 65.00 11 X 21.1 66.29 11.5 Y 21.5 67.51 12 Z 21.9 68.77 12.5 Z+1 22.3 69.99   Z+2 22.7 71.31   Z+3 23.1 72.57   Z+4 23.5 73.83   Z+5 23.9 75.08   Z+6 24.3 76.34   Z+7 24.7 77.60   Z+8 25.1 78.86   To calculate: American size= (diameter-11.54)/(0.83) Japanese size= (diameter-12.67)*3 European size= circumference-40