For a novice knitter it’s pretty easy to become overwhelmed by just going into a yarn shop or crafts store. There are so many choices, choosing which one is best for a particular project can seem daunting. For this reason, most knitters just stick to the exact type (and often the color) of yarn that is suggested in the pattern, but there’s no reason to let fear of failure limit the choice. And it’s not necessary as long as you understand something about yarn weights and how they affect what you are making.
Yarn weight refers to the thickness of the yarn. It ranges from super fine to super bulky. There are six different categories of yarn weights, with specific weights of yarn producing a somewhat predictable number of stitches when a particular sized needle is used. And, of course, the higher the number, the heavier the yarn and the fewer stitches you’ll get per inch.
This is where there is room to play. For instance, if you know that every bulky yarn is going to produce around the same number of stitches, and you have a pattern that uses bulky yarn and say, size 10 needles, then you can buy any kind of bulky yarn and achieve a similar result. Just to be sure, it’s very important for you to knit up a gauge swatch before you begin work on a project that involves sizing, since not all yarns of a particular weight are identical in size. And a difference between twelve stitches per every four inches and fifteen stitches will create a huge difference when you’re talking about something as substantial as sizing a sweater to fit correctly.
Most mass-produced yars use the yarn standards ranking system and will have the number and weight printed right there on the label, or they make it easy to determine the yarn’s weight another way, while others don’t, but they should come with a gauge statement that says something like “24 stitches and 22 rows per four inches on size four needles.” It’s good to have a chart to refer to when attempting to determine what yarn weight you have.
And a little bit of experience and practice will find you more instinctively knowing what size you have by a visual inspection.