I'm adapting a woven shirt for a knitting pattern (next Knitty article, matter of fact), and I'm a little short on yarn. Or shorter than Ms "Buy a skein for luck" prefers to be. SOooooooo, I measured and figured out how much surface area I covered when I knit up one ball of yarn, which was 121 inches. Then I figured out how much surface area the entire shirt takes up. That was a rather amusing little process. I figured the surface area, total, like a giant rectangle, and then subtracted out the areas that aren't knit: armholes and the neck. And I got 880 square inches. I've got the yarn for 960 square inches, so leaving enough yarn for end-darning and the like, I'm going to be running this pretty close.
It's not my fault I had algebra professors who thought knitting math questions were interesting, darn it.
And we won't talk about how my mother-in-law's a retired math teacher who I taught to knit, and how our last knitting discussion devolved to graph paper and radical numbers. No graphing caluculators were involved.
Though I do have one.