There are two separate sizing categories when it comes to hooks, one for standard hooks and one for steel hooks. Knowing which is which is invaluable. The pattern that you’re working from will give you the size hook used to crochet the original design. This is especially important because not only is it the proper size for the type of yarn used, but it also determines the size of the stitch it creates, which determines the gauge. The gauge, in turn, determines the final size of the design.
Hooks come in a wide range of sizes and are labeled accordingly (most standard hooks are labeled with both the U.S. and Continental sizes). The labels are sometimes embossed on the side of the shaft. On other hooks, you may see a raised letter or number on the round end of the shaft. Hooks with padded or “comfort” grips may display this information on the grip.
In the U.S. and Canada, standard hooks are labeled using letters, with B on the small end and S on the large end. In the U.S. and Canada, steel hooks are labeled using numbers. These hooks range from the smallest fraction of a millimeter to about 2 millimeters in diameter. The store gives the most common sizes for steel crochet hooks and standard crochet hooks. Take note that the U.S. and U.K. sizes for steel hooks are the opposite of what you’d expect — the larger the number, the smaller the hook. And the U.K. sizes for standard hooks are a bit backwards, too — the smaller the number, the larger the hook.