Crochet hooks come in a variety of sizes. The size of the crochet hook refers to its diameter. Bigger crochet hooks create looser crochet stitches. Smaller crochet hooks create tighter stitches.

If you look at the packaging on the yarn, it indicates not only the yarn weight but also the recommended hook size. As you increase the weight of the yarn, you also typically increase the size of the crochet hook.

This chart lists the most common crochet hook sizes in millimeters and the corresponding hook size in the US and UK.