As an ongoing part of the series of questions I get frequently asked, How much yarn do I need for X? tops the list right at the same number of times as What kind of needles should I use?
Yarn choices are amazing. There are so many new things out there on the market to try. But this post is to deal with How much not fiber content. Although fiber content can make a difference, weight does make a difference in how much to purchase.
There are generalities I can point to for a quick answer but that doesn't begin to get an accurate number for how much do I need? yarn yardage requirements is a quick link to Lion Brand Company's chart for very basic information for both knit and crochet.
You need to know your gauge with a specific yarn. You need to know how big your project needs to be. You need to know what kind of drape you want your project to have. An example is you don't want a floppy drape-y fiber like alpaca would give you if you are doing a tailored jacket that needs to hold its shape. You also don't want a crisp yarn when your project should be molding to your curves or cuddling baby. All of these have to be taken into account. You should also wash your swatch. Your fiber choice could relax into an ocean sized puddle when gotten wet or shrivel like a prune. Wouldn't it be better to have that happen to a swatch than an afghan or sweater?
But, you say, I'm using acrylic/polyester/super wash wool! Even those fibers can react to water or even just knitting or crocheting in unexpected ways. Please don't ever wash synthetic fibers in hot, hot water and then dry in a hot dryer. You can kill the spring in the fiber.
More will be posted on this topic in the future. This is just to get you thinking. If you have questions, post a comment. It will be addressed.