Sizing System for this style:
Size XS ...... ... Inner Diameter 6mm (4mm thickness)
Size M ............ Inner Diameter Diameter 9mm (4mm thickness)
Size M Slim..... Inner Diameter Diameter 9mm (2.5mm thickness)
Size L ............. Inner Diameter Diameter 13mm (4mm thickness)
Size XL ........... Inner Diameter Diameter 15mm (4mm thickness)