If I’m designing my own sweater and I make a swatch and get it all the way I want it, how do I know how much yarn to buy?
This might be useful. http://www.knittingfool.com/pages/yards1.guest.cfm
Here’s another estimate.
I suggest getting more than you think you’ll need especially if you’re designing your own. You don’t want to run out of a critical color or dye lot. Some places will let you return skeins if they haven’t been wound into a ball or used. Or you could just use leftovers for small projects which is what I do.