I’ve put together a handy guide to translate between metric and USA crochet hook sizes.
To begin, people like myself from country’s such as the U.K. will be used to calling their crochet hooks by the metric size, e.g. 4mm. This is simply the size of the hook shaft in millimetres. Similarly crocheters from other parts of the world such as the US will have a letter/number name for their hook sizes, e.g. G-6. To conclude both a 4mm and a G-6 crochet hook are the same size known by different names. Some crochet hooks will have both terms printed on them which is great. On the other hand some don’t, which is why you might find this crochet hook size guide useful.
What size crochet hook do you need?
If your following a crochet pattern the pattern should clearly state which hook you should be using. But be aware you may not need to use that hook size. After completing your gauge swatch you may need to adjust your hook size to match the tension used in the pattern.
If you are following a stitch rather than a pattern or doing your own thing check your yarn packaging for a recommended hook size.
As a general rule the lighter the yarn weight the smaller the crochet hook will be, likewise the heavier the yarn the larger hook. That being said you don’t have to always use the recommended hook for your yarn. Using a smaller hook will give you smaller, tighter stitches which is useful if you want a stiffer result. Alternatively a larger hook will give you larger, looser stitches which can make your project feel more flexible and also give a softer feel.