Knitting needle sizes range from 2.0mm right up to 25.0mm, and just to make it even more complicated there are three main measurement schemes for needles sizes.
Knitting Needle Conversion
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Knitting Needle Size Conversion Chart
Knitting Needle Size Conversion Chart
Knitting needle and crochet hook size charts - Studio Koekoek
Knitting Needle Conversion Chart
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Knitting Needle Sizes - Knitting Needle Conversion Chart