Crochet hook comparison charts are available online in many sites including Jackie's Crochet & Knitting, Yarn Forward and Lion Brand Yarn. Common conversion charts show hook sizes for the United States, United Kingdom/Canada and millimeters.
Each of these sites offers direct comparison information of metric size, US and UK in simple, one-page charts. Jackie's Crochet & Knitting breaks down the conversions into two categories: crochet hook and steel hook. Yarn Forward includes Canadian sizes, which are the same as UK sizes. Lion Brand Yarns includes US knitting needle sizes on their crochet hook chart. They also have an online library for knitting and crocheting information.