The origins of the US ring size standard are unknown but the standard has been in use since the 19th century, however it is unlikely that its use goes back much further than the mid 19th century as most rings were custom made prior to this date.

The standard uses a numerical scale from 0 to 16, with half and quarter sizes included. Each quarter size indicates an increase in diameter of eight thousandths of an inch (0.008 in) thus making the difference in full sizes 0.032in. This gives an increase of about a tenth of an inch in inner circumference of the ring per full size.

US ring size measurements in inches & millimetres

Size Inner Diameter Inner Circumference   in mm in mm 0 0.458 11.63 1.439 36.55 ¼ 0.466 11.84 1.464 37.19 ½ 0.474 12.04 1.489 37.82 ¾ 0.482 12.24 1.514 38.46 1 0.490 12.45 1.539 39.09 1¼ 0.498 12.65 1.565 39.75 1½ 0.506 12.85 1.590 40.39 1¾ 0.514 13.06 1.615 41.02 2 0.522 13.26 1.640 41.66 2¼ 0.530 13.46 1.665 42.29 2½ 0.538 13.66 1.690 42.92 2¾ 0.546 13.87 1.715 43.56 3 0.554 14.07 1.740 44.20 3¼ 0.562 14.27 1.766 44.86 3½ 0.570 14.48 1.791 45.49 3¾ 0.578 14.68 1.816 46.13 4 0.586 14.88 1.841 46.76 4¼ 0.594 15.09 1.866 47.40 4½ 0.602 15.29 1.891 48.03 4¾ 0.610 15.49 1.916 48.67 5 0.618 15.70 1.942 49.33 5¼ 0.626 15.90 1.967 49.96 5½ 0.634 16.10 1.992 50.60 5¾ 0.642 16.31 2.017 51.23 6 0.650 16.51 2.042 51.87 6¼ 0.658 16.71 2.067 52.50 6½ 0.666 16.92 2.092 53.14 6¾ 0.674 17.12 2.117 53.77 7 0.682 17.32 2.143 54.43 7¼ 0.690 17.53 2.168 55.07 7½ 0.698 17.73 2.193 55.70 7¾ 0.706 17.93 2.218 56.34 8 0.714 18.14 2.243 56.97

Size Inner Diameter Inner Circumference   in mm in mm 8¼ 0.722 18.34 2.268 57.61 8½ 0.730 18.54 2.293 58.25 8¾ 0.738 18.75 2.318 58.89 9 0.746 18.95 2.344 59.53 9¼ 0.754 19.15 2.369 60.17 9½ 0.762 19.35 2.394 60.80 9¾ 0.770 19.56 2.419 61.44 10 0.778 19.76 2.444 62.08 10¼ 0.786 19.96 2.469 62.72 10½ 0.794 20.17 2.494 63.36 10¾ 0.802 20.37 2.520 64.00 11 0.810 20.57 2.545 64.64 11¼ 0.818 20.78 2.570 65.27 11½ 0.826 20.98 2.595 65.91 11¾ 0.834 21.18 2.620 66.55 12 0.842 21.39 2.645 67.19 12¼ 0.850 21.59 2.670 67.83 12½ 0.858 21.79 2.695 68.47 12¾ 0.866 22.00 2.721 69.10 13 0.874 22.20 2.746 69.74 13¼ 0.882 22.40 2.771 70.38 13½ 0.890 22.61 2.796 71.02 13¾ 0.898 22.81 2.821 71.66 14 0.906 23.01 2.846 72.30 14¼ 0.914 23.22 2.871 72.93 14½ 0.922 23.42 2.897 73.57 14¾ 0.930 23.62 2.922 74.21 15 0.938 23.83 2.947 74.85 15¼ 0.946 24.03 2.972 75.49 15½ 0.954 24.23 2.997 76.18 15¾ 0.962 24.43 3.022 76.76 16 0.970 24.64 3.047 77.40