Crochet-Knit.com publishes a crochet hook conversion chart and a steel hook chart, while CrochetCabana.com publishes two hook size charts that show the size and hook number of crochet hooks based on letter assignment. For example, a size B crochet hook is a number 1 hook measuring 2.25 to 2.5 millimeters.
The crochet hook conversion chart on Crochet-Knit.com features metric sizes for both U.S. and U.K. crochet hooks. Using the chart, it is easy to see that a U.S. size C hook is a U.K. size 11 and measures 2.75 millimeters. In addition, CrochetCabana.com offers a Mo-EZ hook chart for larger crochet hooks that are classified by color.