Estimate on how much yarn to buy

I am looking at some yarn on Ebay, but am having a hard time estimating if the yarn will be enough for my project. I realize it depends on the weight of the yarn and the item, but Is there a general 'rule of thumb" to estimate how much is needed for an item? For instance a scarf would be need this many ? grams or yards, a sweater this many grams/yards or an afghan this much?
Thanks for your input.
Denise

Yes it very much depends on it it’s lace, fingering, dk, worsted, or bulky. There’s a chart for the yardage needed in different weights for different items, someone may have a link to it. You could also look up patterns for the same weight and see what they average out to.

Here are a couple of charts that may help but it also depends on the pattern and whether it has bobbles, cables etc. At least it gives you a general ides of yardage but always err on the side of buying too much yarn rather than too little.
http://cache.lionbrand.com/faq/96.html

Here’s a good one. Just remember that they are only an estimate. Yardage can vary depending on the stitch as well as gauge.
http://www.fiber2yarn.com/info/how_much_yarn.htm