Most crochet patterns recommend either a specific brand of yarn to use, or they recommend a yarn weight or yarn type.
The Craft Yarn Council has created a guide which details the types of yarns that fall into each category and will help you pick the right yarn for your project.
The chart also gives a idea of the type of crochet hooks that are generally used for each category of yarn. I say generally, because it will be different for each crocheter. Every crocheter has their own tension; how tight or loose they crochet. Two people using the same pattern, same yarn, and same hook, can end up with two different sized items because one crochets very tight stitches and the other makes looser stitches.
Hook size also depends on how you want your crochet to look. Amigurumi needs a solid fabric style of crochet, so the stuffing doesn’t show through the stitches. That is why I recommend a 2.5mm hook for double knit (DK) yarn in my amigurumi patterns, when the below chart recommends a 4.5mm-5.5mm hook.
Categories of yarn, gauge ranges, and recommended hook sizes.
* GUIDELINES ONLY: The above reflect the most commonly used gauges and needle or hook sizes for specific yarn categories.
** Lace weight yarns are usually crocheted on larger hooks to create lacy, openwork patterns. Accordingly, a gauge range is difficult to determine. Always follow the gauge stated in your pattern.
*** Steel crochet hooks are sized differently from regular hooks–the higher the number, the smaller the hook, which is the reverse of regular hook sizing
The Yarn Weight Guide is not a direct interchangeability chart. It is possible to interchange two yarns in one weight category, however I recommend that you always crochet a gauge swatch to make sure that the size of your finished item will not be impacted. There are also other factors to consider such as the yarn softness and how it drapes.
Have you experimented with different types of yarn? The next time you go yarn shopping challenge yourself to try something new.