How do you figure out how much yarn is necessary for a particular pattern?
In general, each pattern usually indicates how much yarn that particular project needs. It is a good idea to buy yarn that suggested by the pattern. If it's not possible then look at the yarn's label. Label will have all information we need to know about yarn, eg. yarn gauge, recommended needle and crochet sizes, types of wool, and so forth. It will tell you how long each ball of yarn has in yards and meters. Then you just have to decide how many ball you need to buy according to the pattern.
I want to make a scarf using a pattern that requires 250 yards. I found a ball of yarn I want and it has only 100 yards long. So I have to buy 3 balls of yarn in order to accommodate my pattern. This is the reason why, we, the knitters have so many left over yarn!
I hope this helps.
Best of luck and have a very happy holiday!
ps. sorry for answering your question late. I missed the email and holiday is just too crazy!