ring size

   a measure of the inside diameter or inside circumference of a ring (the kind worn on a finger). A variety of ring sizing systems are used in various countries. In the U.S., a ring of size n has an inside circumference of 1.43 + 0.102·n inches, or about 36.3 + 2.60·n millimeters. (There is some variation, because U.S. ring sizes have never been standardized). In Britain, traditional ring sizes are stated as letters A, B, etc.; if we replace the letters by numbers n (A = 1, B = 2, etc.), then a ring of British size n has an inside circumference of 36.25 + 1.25·n millimeters, or about 1.43 + 0.049·n inches. A difference of 1 U.S.size thus corresponds rather closely to two letters in the British system. In Japan, sizing is by the inside diameter in increments of 1/3 millimeter; a ring of Japanese size n has an inside diameter of (n + 38)/3 millimeters and an inside circumference of 39.8 + 1.047·n millimeters. There is an international standard (ISO 8653) defining the ring size to be the inside circumference in millimeters, minus 40. Rings are now sized by this standard in most of Europe, so a ring of European size n has an inside circumference of exactly 40 + n millimeters. (The British scale is aligned with the European scale, with British size C corresponding to European size 0 and a difference of four British letters corresponding to 5 European sizes.)