The amount of yarn needed for a crochet pattern depends on many factors, such as the dimensions of the finished product, the thickness of the yarn and the size of the crochet hook. Most crochet patterns come with instructions for purchasing the correct yardage of yarn.
If a pattern does not come with yardage information, or if someone is creating their own pattern, there are some options for determining the approximate amount of yarn needed.
Some yarn manufacturers, such as Lion Brand Yarns, have charts on their websites that list projects by type and size and give the corresponding amount of yarn required. There are also online calculators that help determine the number of yards needed for a project of a certain size. In general, it is good to estimate on the high end when purchasing yarn for a project, so as not to end up with too little yarn.