Convert / Rings
Ring size converter
US, UK letters, EU / ISO, Swiss and inner diameter. Ring sizes are defined by one measurement, so this conversion is exact.
Formula only. We have not checked brand charts for this category yet, so there is no brand column and no wearer panel here.
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How this converter decides
Every ring system is a way of writing the inner circumference. ISO 8653 sizes are that circumference in millimetres; a US size is (circumference minus 36.5) divided by 2.55; UK letters start at 40 mm for C and add 1.25 mm per letter; Swiss, Italian and Spanish sizes are the circumference minus 40. Measure the inside diameter of a ring that fits and start from there.