Here you have a chart with the different ring sizes equivalents in mm. You can measure your finger diameter or check the inside measure of one of your rings to know your size. 

SISTEMA MÉTRICO UE TALLAS US TALLAS
13,05 mm 1 1
13,69mm 3 2,5
14,33mm 5 3,5
14,96mm 7 4
15,60mm 9 5
16,24mm 11 5,75
16,87mm 13 6,5
17,51mm 15 7,5
18,15mm 17 8

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