| British Ring Size |
Inside Diameter (mm) |
Inside Diameter (in) |
European Size |
American Size (approx) |
| A | 12.065 | 0.4750 | - | - |
| - | 12.096 | 0.4762 | 38 | - |
| A ½ | 12.263 | 0.4828 | - | - |
| - | 12.344 | 0.4860 | - | 1 |
| - | 12.414 | 0.4887 | 39 | - |
| B | 12.459 | 0.4905 | - | - |
| B ½ | 12.657 | 0.4983 | - | - |
| - | 12.733 | 0.5013 | 40 | - |
| - | 12.751 | 0.5020 | - | 1 ½ |
| C | 12.852 | 0.5060 | - | - |
| C ½ | 13.051 | 0.5138 | 41 | - |
| - | 13.157 | 0.5180 | - | 2 |
| D | 13.246 | 0.5215 | - | - |
| - | 13.369 | 0.5263 | 42 | - |
| D ½ | 13.444 | 0.5293 | - | - |
| - | 13.564 | 0.5340 | - | 2 ½ |
| E | 13.640 | 0.5370 | - | - |
| - | 13.678 | 0.5389 | 43 | - |
| E ½ | 13.838 | 0.5448 | - | - |
| - | 13.970 | 0.5500 | - | 3 |
| - | 14.006 | 0.5514 | 44 | - |
| F | 14.034 | 0.5525 | - | - |
| F ½ | 14.232 | 0.5603 | - | - |
| - | 14.324 | 0.5639 | 45 | - |
| - | 14.376 | 0.5660 | - | 3 ½ |
| G | 14.427 | 0.5680 | - | - |
| G ½ | 14.625 | 0.5758 | - | - |
| - | 14.642 | 0.5765 | 46 | - |
| - | 14.783 | 0.5820 | - | 4 |
| H | 14.821 | 0.5835 | - | - |
| - | 14.961 | 0.5890 | 47 | - |
| H ½ | 15.019 | 0.5913 | - | - |
| - | 15.189 | 0.5980 | - | 4 ½ |
| I | 15.215 | 0.5999 | - | - |
| - | 15.279 | 0.6015 | 48 | - |
| I ½ | 15.413 | 0.6068 | - | - |
| - | 15.597 | 0.6141 | 49 | 5 |
| J | 15.608 | 0.6145 | - | - |
| J ½ | 15.806 | 0.6223 | - | - |
| - | 15.916 | 0.6266 | 50 | - |
| K | 16.002 | 0.6300 | - | 5 ½ |
| K ½ | 16.200 | 0.6378 | - | - |
| - | 16.234 | 0.6391 | 51 | 1 |
| L | 16.396 | 0.6455 | - | 6 |
| - | 16.552 | 0.6517 | 52 | - |
| L ½ | 16.594 | 0.6533 | - | - |
| M | 16.789 | 0.6610 | - | - |
| - | 16.815 | 0.6620 | - | 6 ½ |
| - | 16.871 | 0.6642 | 53 | - |
| M ½ | 16.988 | 0.6688 | - | - |
| N | 17.183 | 0.6765 | - | - |
| - | 17.189 | 0.6767 | 54 | - |
| - | 17.221 | 0.6780 | - | 7 |
| N ½ | 17.381 | 0.6843 | - | - |
| - | 17.507 | 0.6893 | 55 | - |
| O | 17.577 | 0.6920 | - | - |
| - | 17.628 | 0.6940 | - | 7 ½ |
| O ½ | 17.775 | 0.6998 | - | - |
| - | 17.826 | 07.018 | 56 | - |
| P | 17.971 | 0.7075 | - | - |
| - | 18.034 | 0.7100 | - | 8 |
| - | 18.144 | 0.7143 | 57 | - |
| P ½ | 18.169 | 0.7153 | - | - |
| Q | 18.364 | 0.7230 | - | - |
| - | 18.453 | 0.7265 | - | 8 ½ |
| - | 18.462 | 0.7269 | 58 | - |
| Q ½ | 18.562 | 0.7308 | - | - |
| R | 18.758 | 0.7385 | - | - |
| - | 18.780 | 0.7394 | 59 | - |
| - | 18.872 | 0.7430 | - | 9 |
| R ½ | 18.956 | 0.7463 | - | - |
| - | 19.098 | 0.7519 | 60 | - |
| S | 19.152 | 0.7540 | - | - |
| - | 19.279 | 0.7590 | - | 9 ½ |
| S ½ | 19.350 | 0.7618 | - | - |
| - | 19.416 | 0.7644 | 61 | - |
| T | 19.545 | 0.7695 | - | - |
| - | 19.736 | 0.7770 | 62 | - |
| T ½ | 19.741 | 0.7772 | - | - |
| - | 19.761 | 0.7780 | - | 10 |
| U | 19.939 | 0.7850 | - | - |
| - | 20.054 | 0.7895 | 63 | - |
| U ½ | 20.137 | 0.7928 | - | - |
| - | 20.168 | 0.7940 | - | 10 ½ |
| V | 20.333 | 0.8005 | - | - |
| - | 20.372 | 0.8020 | 64 | - |
| V ½ | 20.531 | 0.8083 | - | - |
| - | 20.599 | 0.8110 | - | 11 |
| - | 20.690 | 0.8146 | 65 | - |
| W | 20.726 | 0.8160 | - | - |
| W ½ | 20.925 | 0.8238 | - | - |
| - | 21.008 | 0.8271 | 66 | 11 ½ |
| X | 21.120 | 0.8315 | - | - |
| X ½ | 21.318 | 0.8393 | - | - |
| - | 21.326 | 0.8369 | 67 | - |
| - | 21.412 | 0.8430 | - | 12 |
| Y | 21.514 | 0.8470 | - | - |
| - | 21.645 | 0.8522 | 68 | - |
| Y ½ | 21.712 | 0.8548 | - | - |
| - | 21.819 | 0.8590 | - | 12 ½ |
| Z | 21.908 | 0.8625 | - | - |
| - | 21.963 | 0.8647 | 69 | - |
| Z ½ | 22.106 | 0.8703 | - | - |
| - | 22.225 | 0.8750 | - | 13 |