Things You'll Need
When you've got an idea for what you want to make but not a specific pattern in mind, it can be hard to tell how much yarn to buy. Or perhaps you've spotted the most beautiful yarn in your local yarn store -- but how do you know how much to get? By doing some educated guesswork or making use of patterns, you can make sure you don't have to stop in the middle of a project to acquire more yarn.
Decide what you want to make. Should it be a sweater or a blanket or a hat? Check the weight of your chosen yarn. If it's a lighter gauge, you'll need more yarn since you'll be crocheting smaller stitches. For a heavier gauge, you would use less.
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Learn your yarns. Crocheting a laceweight baby blanket uses approximately 2,000 yards, and a bulky weight baby blanket uses approximately 1,000 yards. Crocheting with worsted and sport weight yarns falls somewhere in between.
Extrapolate from those estimates to guess how much you'd need for other projects. If a baby blanket is 20-by-20 inches, the area of an adult sweater would be a little over twice that, so you would need a little more than twice as much yarn.
Working From a Pattern
Check your pattern, if you already have one in in mind, as it should tell you the number of yards you'll need. Some patterns, however, give you only a certain kind of yarn, and tell you how many skeins of that particular yarn to buy, which isn't very helpful if you want to use a different brand. In that case, check to see how many yards the recommended yarn has per skein, and make sure you buy the same amount.
Decide whether you want to use a different weight of yarn than the one the pattern calls for. That will change the number of yards you need -- of course, it will also change the size of the finished object, so that a hat crocheted in a sport weight yarn instead of a worsted weight will be smaller than the pattern says it will be, even if you use the same size crochet hook. Again, if you're using a lighter weight yarn, you'll need more yarn than the pattern calls for; if you're using a heavier yarn, you'll need less.
Crochet a few inches of the pattern, and then unravel to see how much you've used. This will give you a more precise estimate of the yarn you will require. For example, crocheting 4 inches of a baby blanket uses 200 yards. If the baby blanket is going to be 24 inches long, then you will need 1,200 yards.
When you're estimating, always get a little more than you think you need. It's better to have too much yarn than too little.
For those who are very good at math, make a swatch by crocheting a 4-by-4-inch square. Unravel the swatch and measure how much yarn you used. Then use a sample garment, like a favorite sweater, and your measuring tape to determine the area of the finished sweater. Then multiply how many yards of yarn you'll need to crochet that area.