Goodness, even in knitting there are weight issues! Is there no escape? The answer is a resounding "no." But fear not, yarn does not judge or discriminate...all weights are beautiful. You just need to know what weight is appropriate at what time. As you can clearly see in the photograph below, that skinny minnie lace weight yarn is no good for your warm cuddly scarf and that super bulky yarn would never work for an intricate pair of socks. Shown side by side, there is a clear difference in the yarns, even if they are the same color and fiber content.
Yarn weight is important for the success of your project. What it really comes down to is the gauge you are attempting to achieve, which is why swatching is so important. Any good knitting pattern will tell you what gauge was used to complete the project, which is simply stating how many stitches there are per a prescribed measurement. Often in the United States you see it as stitches per inch or perhaps stitches per four inches. I will do a more detailed tutorial on swatching, but here is a nutshell summary. If you use the needle size and yarn a pattern calls for and have more stitches than outlined, you should move up a needle size. If you have fewer stitches, move down a needle size. If you have the same number, you're good to go. There are a number of reasons gauge can be off, one of which is your yarn weight.
One of the important tidbits of information a pattern gives you in achieving the appropriate gauge is what yarn was used to knit the project. You can use the same exact yarn, or you can substitute in your own choice...which is where the yarn weight begins to matter a lot. If you're a newbie at this, you should probably stick with a yarn that is as similar to the yarn in the pattern as you can get. To do this, you need to know what weight, or thickness, you are dealing with. There are thicknesses that are assigned a number, 0-7, zero being the thinest. Just because having one designation is not enough, there are names associated with thickness, as well. These names start at "lace weight" and continue to "super bulky." An excellent chart of these designations is found on www.yarnstandards.com and has been copied below for your convenience.
But these designations, whether numbers or names, include a small range of sizes. Of course two lace weight yarns, spun by two different companies, will be a little different. (Again, that is why swatching is so important. Even if you sub in a lace weight for another lace weight, you may gain or loose a stitch and need to adjust your needle size.) The most scientific way to discern exact yarn weight is to, you guessed it, weigh it. Oddly enough, however, the weight we have been discussing is different from the weight listed on your yarn label. (Stick with me.) The weight designations that tell you if you are woking with a DK or a bulky yarn would probably be more accurately called a "thickness" designation. But the actual weight of your yarn can be useful, too.
When you look at most yarn labels, you will see both the yardage and the weight listed. For easy comparison, you can do a little math to get the yard (or meter) per oz (or gram). Using the photographed label below, let's try it out. The label says there are 220 yards in this particular hank of yarn. It weighs 100 grams. Take 220 yards divided by 100 grams to yield that there are 2.2 yds/g. This is a good, quick way to compare yarns with the same fiber content. (They must be the same fiber content for this comparison because different fibers have different densities. Makes sense, right? But that means two equal thickness yarns of different fiber content can have very different weights.)
Why does this matter? Well, let's pretend the pattern I have calls for the beautiful Cloudborn Fiber featured above, but I do not want to use this yarn. I was browsing my yarn shop and fell in love the Malabrigo prussia blue yarn and want to use it instead. Both yarns are a superwash merino wool, so are the weights close enough for substitution? I look for a number on the Malabrigo, but it does not have a standard designation or symbol, but I do not panic. Using the label, I whip out my phone and check the numbers. 335 yards divided by 100 grams is 3.35 yards/gram. This is a little different from the 2.2 yds/g of the Cloudborn. While I could adjust my needle size enough to make up the difference, it may change the pattern too much for my comfort level. There is a full yard more per gram of the Malabrigo over the Cloudborn, meaning the Malabrigo is more a light worsted weight yarn than a worsted weight ( a #3 opposed to a #4). The answer: I keep shopping.
Okay, that Malabrigo was not a good substitution, so what now? Further browsing and I stumble upon a beautiful wool KnitPicks yarn. Using the label, let's see if this yarn would be a good weight. It has 110 yards per 50 grams, which is 2.2 yds/g. That is an exact match for weight! Hooray! Also, a quick look at the label reveals both yarns are a #4 worsted weight. Perfect. (You don't have to give up a substitution if one yarn is a #3 and one is a #4 weight. It could happen that the #3 is at the top end of the light worsted weight range and the #4 is at the low end of the worsted weight range and that the substitution can, in fact, easily be made.)
It can be fun to knit with different weights and once you get comfortable, you can start making more daring substitutions. You will find that different weights give you different affects and can really make a striking difference in the look of a project. When you are following a pattern, if you stray too far from the prescribed weight you may have to make adjustments to the pattern itself, which may or may not work out depending on stitch counts. In the example I gave earlier, it would never work out to substitute a bulky weight for a lace weight. While an extreme example, it illustrates that need to pay attention to gauge, which is directly affected by weight.
Obviously, thicker yarns knit on larger needles tend to be much faster projects than lighter weight yarns knit on small needles. This brings me to my last point- needle size. In the chart earlier, you can see different weights call for different needle sizes. Of course there are exceptions, like how lace is often knit on larger needles. But, as with my outrageous suggestion of using a super bulky yarn for a delicate sock pattern, it just would not make sense. You could never use a size 1 needle on a super bulky yarn. Staying within the recommended needle size for your yarn yields the most attractive stitches. Someone else figured it all out so you don't have to. It's worth paying attention to their suggestion.
It is fun to explore the yarn attribute of weight and I hope that you do.