Ring size converter
Convert a ring size between the US, UK, European and Swiss systems and the inner diameter and circumference in millimetres — the numbers a jeweller and a 3D model actually use.
US size = (diameter − 11.63 mm) ÷ 0.8128; ISO = inner circumference in mm; UK letters from A = 37.8 mm circumference in 1.25 mm steps. Charts differ by up to a quarter size between makers, and wide bands fit tighter — have a jeweller size you before ordering.
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How it is calculated
US and Canadian sizes follow a formula: inner diameter = 11.63 mm + 0.8128 mm per size, so a US 7 is 17.3 mm across.
European sizes (ISO 8653) are simply the inner circumference in millimetres; Swiss sizes are that minus 40.
UK and Australian letters start at A = 37.8 mm circumference and rise 1.25 mm per letter, with half sizes between.
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