Crochet hooks range in size from 2.25 to 19 millimeters, according to the Craft Yarn Council website. In the United States, many manufacturers use both letters and numbers to label the size of crochet hooks, so consumers should look closely at all packaging to discern the size in millimeters.
Differences in product labeling confuse consumers, according to the Craft Yarn Council website. It advises manufacturers to use both labeling techniques to show size by letter, number, and millimeter to prevent confusion. Due to lobbying by the Craft Yarn Council, many manufacturers will implement changes to promote the clear labeling of crochet hook size in new product packaging and new crochet patterns.