Estimating how much yarn to buy for a knitting or crochet project can be tricky, especially if you are substituting handspun yarn for commercial. There are a couple ways to figure this out depending on if you are using a pattern or making it up as you go along. Either way I recommend buying at least 5-10% more than you think you need. Who wants to run out of yarn when you're almost finished with your project? Yup. Exactly.
If you are using a pattern:
How many yards does the pattern call for? Figure out the total yardage you'll need: (number of skeins) x (yardage per skein) = total yardage
Here's a handy chart from Knitting Patterns For Dummies for estimating yardage for yarn projects.
Estimated Yardage of Yarn for Projects Yarn Weight Category Stitches per Inch Yards Needed for a Hat Yards Needed for a Scarf Yards Needed for an Adult Sweater 1 Superfine 7 to 8 300 to 375 350 1,500 to 3,200 2 Fine 6 to 7 250 to 350 300 1,200 to 2,500 3 Light 5 to 6 200 to 300 250 1,000 to 2,000 4 Medium 4 to 5 150 to 250 200 800 to 1,500 5 Bulky 3 to 4 125 to 200 150 600 to 1,200 6 Super bulky 1.5 to 3 75 to 125 125 400 to 800
If you are freestyling and/or using handspun yarn:
With handspun yarn this can be tricky. A skein of handspun yarn usually weighs anywhere between 3-4 oz. with the average being 3.5 oz. A super bulky handspun yarn can be anywhere from about 25-60 yards per skein. Here's a trick - Let's say you are making a scarf. Use a kitchen or postal scale and weigh a scarf that is about the same size and yarn weight as your project will be when finished. If your scarf will weigh 2 lbs. and you are using 3.5 oz. skeins, then you'll want to calculate 16/3.5 x 2. You would need 9.15 skeins of yarn.
Here's more info in my previous post about working with handspun yarn. Of course, this is not an exact science. Needle or hook size will vary as well and with handspun yarn it's often better to use bigger hooks or needles than you would with commercial or mill spun yarn. This allows for the natural variances in thickness and twist in the yarn. Try a swatch first and see how it goes. Many spinners will sell sample sizes or have patterns for their yarn, so if you are not sure how much to buy don't be afraid to ask.