If you are new to crochet and you are just trying to figure out hook sizes, we have something for you.
Have you ever been in a situation where you noticed that the size 8 steel hook you have is different from the size 8 hook being used in the pattern you are following? Well, it turns out that even if they are both number 8, they might not be the same size. 8 might refer to IMIA or Japanese size 8, and it is different from 8mm or US 8. Think of it this way: 8 feet is not the same length as 8 inches.
Crochet hooks, and knitting needles too, are labeled with sizes based on different standards or should I say, systems. These could be UK, US, Metric, or Japanese.
I made a compilation of various hook sizes with their corresponding conversions so you will be aware of their equivalent sizes across various systems.
I also made a compilation of knitting needle sizes.
I hope this helps you for now (or at least prevents you from guessing hook/needle sizes).