A Guideline to the Amount of Yarn Needed

[Patsys]

Is there a chart or listing with a guideline to the amount of worsted weight yarn needed to knit an afghan to fit a twin sized bed?

[suzeeq]

Take a look at patterns that are approximately that size and figure it out from there. If an afghan is 40x50 that would be 2000 sq inches, and if you want it 40x70 that's 2800 square inches which is 1.4 times larger. So if the 40x50 takes 2400 yards, the larger one would need about 3360 yards. You might need more if you use cables or ribbing as they draw in, but you might need less if you use a larger needle, like a size 10 or 10½.

Another way to figure it would be to cast on about 40 sts and knit 40 rows. Measure the sq inches in that, then measure how many yards it took (simple estimate is measure how much yarn for one row times X rows plus add an extra row to account for the cast on.) and figure it out from that.

[fatoldladyinpjs]

A lot of it depends on what size needles you're going to use, too. I think you use less yarn with larger size needles, like a 13. If you're using an acrylic for this, I suggest you buy the large one pound skeins with no dye lot. Caron One Pound yarn would be good for this. I buy mine at Joann's Fabrics. There are several places online where you can buy this also. Walmart offers it online but not in their stores. However, they have limited colors.

http://www.caron.com/color_cards/cc_onepound.html
The prints here are only 10 oz skeins, but they're specially designed to compliment any One Pound solid color.

[Jan in CA]

Here's a yardage guide, but it only lists a standard afghan not a whole blanket. You might be able to get an idea if you know the measurements you want.
http://www.fiber2yarn.com/info/how_much_yarn.htm