Like bebedechocolate said, you need to check the yardage of the original yarn. I will give you an example from the shawl I'm getting ready to knit:
I'm going to knit Woodland Shawl. It calls for 1 skein Lana Grossa Mega Boots Stretch (100 grams, 460 yards). So I know that 100 grams of this particular yarn has 460 yards.
I'm using Knit Picks Palette. Each ball of Palette is 50 grams and 231 yards. So I will need to but 2 balls to equal 100 grams and 462 yards. I bought 2 extra though, because they are cheap, I like the color, and I'm paranoid of running out!
Hope that helps.