When you go into a yarn shop or crafts store, it's easy to be overwhelmed by all the choices and not know which yarn is right for your project. Most people stick to the exact type (and even the color) of yarn suggested in the pattern, but it's handy to know how to adjust knitting instructions if you're drawn to a different type of yarn. While every yarn isn't perfect for every project, learning about yarn weights will allow you a lot more creativity in your choices.
The Standard Yarn Weight System
Yarn weight refers to the thickness of the yarn. It's a range, from super fine to super bulky. There are six different categories of yarn weights, and according to the Craft Yarn Council, specific weights of yarn should produce a somewhat predictable number of stitches when using a particular-sized needle.
The higher the number, the heavier the yarn and the fewer stitches per inch you will get but ply doesn't always correlate to the weight of a particular yarn.
Why Standards Matter
This is where knitting gets fun. If you know that every bulky yarn is going to give you around the same number of stitches (in this case, 12 to 15 stitches in 4 inches on size 9 to 11 needles) and you have a pattern that uses bulky yarn and size 10 needles, you can buy any kind of bulky yarn and get a similar result.
It is, of course, essential that you knit up a gauge swatch before you start work on a project that involves sizing because not all yarns of a certain weight are exactly the same. The difference between 12 stitches per 4 inches and 15 stitches is still pretty significant when you're trying to make a sweater fit.
Determining Yarn Weight
Most yarn manufacturers make it easy for you to determine the weight of a particular yarn. Many of the mass-produced yarns use the yarn standards ranking system and will have the number and weight printed right on the label.
Other manufacturers don't make it as easy, but they should have a gauge statement that will say something like "24 stitches and 22 rows per 4 inches on size 4 needles." If you know a little bit about yarn weights (which you will when you consult the chart below), you'll know that the yarn in question is sport weight.
If you're still having trouble figuring out a yarn weight, the clerks at your craft store should be able to answer any questions you have. You also can search for specific brand yarn weights online—most manufacturers provide this information.