Yarn weight. It's something that confuses many, regardless of where they are in their stitching ability - novice, expert, or somewhere in between - yet it's a huge part of one's stitching success.
The lingo can get confusing sometimes. Even as the industry is regularly use the standardized weight classification system, there's still a lot of confusion out there. Just as in needle and hook size, the area of the world in which you live may affect the vocabulary and standards you use. This isn't a big problem, as long as you realize it and are able to slow down and sort through the information you do have. It's most notably an issues if yo'u're using yarn that's older than the classification system or from another country.
Darn Yarn is organized by yarn weight. When we say "yarn weight" we aren't referring to how many grams or ounces are in the unit, but rather how thick or thin the diameter of the yarn is. We chose this as our starting point for organization because most stitchers start with an idea, project, or specific need - "Excuse me, I want to make a _____". While this can be somewhat open ended, at the same time, most of us know what the end result should feel like; heavy or bulky, light and airy, squishy, open and lacy, or comfortably warm but not bulky. We want (usually) our socks to be lightweight, our sweaters somewhere in the mid range, our mittens usable with out bulk, and our shawls wrappable yet lightweight .
This is where familiarity with the different yarn weights comes in handy. Basically, the thinner weight yarns work up more stitches per inch/cm on smaller hooks or needles while wider diameter yarns usually have fewer stitches per inch/cm and use bigger hooks or needles. There is, of course, a lot of leeway in this regard, but it's a starting point that can help us when trying to understand the differences between the yarn weight categories, and when talking about patterns, yarns, and projects with fellow stitchers.
Below is a chart I've compiled from a number of other sources (listed at end). As you can see, it's quite easy to get confused when the same words can be used for multiple weights and when stitches per inch and and recommended hook or needle sizes have overlapping ranges.
Neighboring weights often overlap. That overlapping range is the grey zone where people often get confused, stating something like, "This looks rather thin for _____" or the exact opposite, "Isn't this a rather heavy ______ weight?". But, and here's the good bit, that's also where there's a lot more freedom when it comes to yarn substitution!
For example, I'm working on a scarf that calls for DK weight/ US 6 needles, 20 sts / 4". I'm using both Sport (2) and DK (3) weight yarn together in this project. The Sport weight yarn in this project has a recommended gauge of 20 - 24 sts on US 4 - 6 on it's label - which is within range of the pattern and the DK standards above, as well as the Sport standards in which it is categorized by the yarn company. Because this project is a shawl, the different yarns are similar enough and both fit the pattern requirements, so I'm not concerned about the difference in stated weights. And so far, it's worked out fine. (Photo left: green is sport weight, pink is DK.)
If you need to substitute yarn in a pattern, sometimes these grey areas around the edges of the categories allow you to use a yarn that may be categorized as a different weight than the one called for in the pattern. If the yarn company's recommended stitches per inch gauge and hook or needle size on the label matches the stitches per inch gauge and hook or needle size in the pattern, you may be more successful than if you simply substituted yarn based on the weight category.
This is even more true when you're looking to find a substitute that's the same fiber as the recommended yarn. Some blends and fibers act differently when worked up, the yarn companies know this and take that into account when categorizing their yarns. So think outside of the box a bit when substituting and don't be afraid to check a few yarns in the neighboring categories - and of course, swatch!
Always swatch when substituting yarn in a pattern where fit is critical! The extra time taken at the start will be worth it. If you've ever had to frog (unravel a piece back to the start) something after working half of the pattern, you know what a time saver swatching can be. Swatching can also be important if the pattern gauge and recommend hook/needle size are conflicting (really large or small hook/needle for yarn weight ) and you are uncertain as to why. Many knit shawls, for example, use Fingering (1) weight yarn and US 5 / mm needles while sock knitters use the same weight yarn and much smaller, more "gauge appropriate" needles. So a shawl pattern using Fingering (1) weight yarn and US 8 needles might make one wonder - unless it's a very open and lacy piece.
Understanding yarn weight classification is also useful when stash-busting. I know I'm not the only one with unlabeled yarn in my stash. This is where the WPI, wraps per inch, comes in handy. Spinners use WPI as one of the ways to determine the weight of their finished yarns. So, if you're a spinner, it's likely that you already know how to use and figure out a yarn's WPI. For those who don't, WPI is exactly what it sounds like; simply wrap your yarn around a ruler and count how many wraps fill in an inch.
There are WPI gauges available, but a good old wooden ruler works just as well. Don't tug or pull it to tightly, and try not to wrap it to loosely either. It's not an exact science, and with practice you'll get the hang of it. Again, as you can see in the chart, there's a bit of leeway, but it's a relatively easy way to organize your stash and determine which random favorite yarns can be used together with a fairly decent success rate.
Knowing a little about the yarn weights can certainly help any stitcher choose yarn with a bit more confidence. Just remember, it's not always a sure thing. After all, there are those overlapping edges between weights; even the most experienced stitcher may still be surprised by the way a yarn behaves from time to time.