Ring sizing in Europe is typically performed using on of three sizing standards based on metric measurements. Two of the systems derive the size from the internal circumference of ring, and the other is based on the internal diameter of the ring.
Same Metric Measurement = Different Metric Size?
So how can there be two size systems based on metric measurement of circumference? The answer is not too complicated. If anything it is an editorial on human nature. In the US ring sizing system, an average lady has a finger size of approximately 6.5. In the metric system this size corresponds to a size of approximately 53 (circumference equals 53.1mm). Imagine a lady who wears size 7 sneakers, and size 8 dresses being told that her finger size is 53! The number sounds a little too big for a svelte lady accustomed to small numbers. Thus the secondary Swiss ring sizing system that takes the metric measurement and subtracts 40 from it to end up with a more comfortable: size 13. The basic circumference measurement in millimeters is outlined in ISO standard 8653:1986.
Circumference vs Diameter
While the ISO standard for ring sizes is based on the internal circumference, there is another ring sizing scale based on the internal diameter. This scale has been called the German sizing scale, as it sees some use in that country. This measurement is perhaps the most useful of all foreign systems, as any circular ring may be measured for it's internal diameter to provide the size. The measurement should be measured to the tenth of a millimeter, for it to have accuracy.
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