Need help determining how much yarn needed
Making a simple stockinette scarf. One skein of the alpaca yarn I want to use is approx. 220 yards. Label says gauge is 5 stitches per inch; use 68 needle size. Don't know how to calculate how many skeins for a scarf approximately 10 inches wide and 72 inches long. Thanks in advance for the help. If you can walk me through the calculations too, so that I learn, that would be great.

That may be enough if you use the larger needle, or even a size 9 or 10, but for a regular 6" scarf. Only you can know how much you use. You could CO 20 sts, knit for 4 inches, measure the width and length to see how many square inches that is. Then measure how much yarn that took and divide 720 sq inches by the sq inches you got and multiply that by the yarns it to you to make your sample. 10" is pretty wide though, most scarves are generally 68" wide. So casting on the 20 sts should let you see how wide that turns out and figure out if you actually do want it 10" wide.

thanks Suzeeq. I actually am very sure of the width (just made one with bulkier yarn for a friend and that width and length are perfect). but I'm unclear why I have to knit a swatch. I can't do a good estimate using a formula? I know that when I bought the bulkier yarn, the clerk in the store used a calculator to determine exactly how much I'd need. Whatever she did sure worked. I had enough, and a little left over.

The formula is determined by your gauge and tension  how much yarn it takes you to knit something x by y size. You might call that store and ask the clerk how she did it. I don't know of any preset formula.

maybe you were making a scarf by a pattern that the clerk knew? If you change the yarn in a pattern, for example, you can often calculate how much you need by the legth per 50 g and the stitch count.
I do not know any yarn formular, either. If I really wanted to know ahead of time and had no pattern to start from, I might find 2 ways to get close to an answer: look  for example on ravelry  for scarfs of the same yarn and the size they got with a certain amount. Every knitter IS different, but it will give you an idea. the other thing: with a scarf you know how many stitches you have across and how many rows per 4 inch, once you made a swatch. Now I would knit a swatch 4 inches long in the size of the scarf (or just knit the scarf for that long, really). Now I figure out the WEIGHT of yarn I used by now. the knitting has the needles in. Either weigh a needle beforehand and deduct the weight or weigh the skein before and after to find out the difference. Now that is your usage for 4 inches of scarf. Now you know how many inches you want it to be and multiply accordingly. Warning: the lighter the yarn is the less acurate your calculation may be because small differences make a big change on the calculations. But if you know the width and length you really need, then just reserve yarn at the yarn shop, or buy extra for safety, or buy stuff you can bring back (like at my yarn shop: they take back unused balls) 
Thanks hyperactive. I'm still pretty confused, but you gave me lots to think about. I have a wonderful shop that takes back anything, so I always buy too much on purpose to make sure I'll have enough. I think I need to go over there again and speak with the person who set me up so well last time ... but this time ask her to walk me through the mental math she's doing to estimate how much yarn I need. Again, thank you for your help.

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