I am putting this up mainly so that I can link to it on the Pattern Support page, to assist with questions about yarn substitutions.
But now that we are here, why not chat bout yarn weights?
It's a topic that confuses even experienced knitters. And understandably so! Because there is a headache-inducing degree of subjectivity to it. So if you are searching for terms such as 'standard yarn weights' on the internet and finding conflicting or overlapping sets of 'standards' do not be alarmed - that is just how it is. The above chart is not intended as a definitive system to overrule all other systems. It is simply my system, and if you are using my patterns it might be handy to have it as a reference.
The other important thing I wanted to note, is that yarn weights are really a continuum. As far as exact meterage, more often than not a yarn will fall in between categories, in which case it will usually be labeled according to the heavier category (even if in actual meterage it is closer to the lighter category!). For example: a yarn with a meterage of 425m/100g will be labeled as Fingering weight. A yarn with a meterage of 375m/100g will be labeled as Sport. A yarn with a meterage of 240m/100g will be labeled as DK. A yarn with 190m/100g will be labeled as Aran.
The above is important to be aware of. Because when choosing a suitable yarn substitute for a pattern, it is not merely a question of whether you are able to meet gauge with this alternative yarn. And neither is it enough to ensure that the subsitute yarn belongs to the same weight category. It is crucial to check the actual meterage of the yarn you are considering substituting with, and to compare it to the meterage of the yarn used in the pattern.
For example, let's say a pattern calls for a yarn with a meterage of 200m/100g, which makes it a DK. You decide to substitute with a different DK yarn - only this one happens to be 250m/100g. Now, if you think about it, that is actually a significant difference in meterage! And this variation will certainly affect the characteristics of your fabric. Namely: The greater the meterage per 100g, the thinner the yarn, and the less dense (i.e. more drapey, less structured) your resulting fabric will be (assuming you are meeting the gauge stated in pattern). And while in some instances that may actually work well, in others it may compromise the garment you are knitting. It really depends on the pattern, and on the designer's intent.
So how do you substitute yarn, while ensuring that the fabric you create stays close to what the pattern intends?
Well, the obvious answer is: Look for yarn with meterage that matches as closely as possible that of the yarn used in the pattern. And if the yarn you like is off by a bit, consider what effect that will have on your fabric, and whether this is compatible with the type of garment you are looking to knit. So, for example: If you are knitting a flowing drapey shapeless sweater, going with a slightly thinner yarn than specified in the pattern could work, enhancing the qualities inherent to the design. On the other hand, going with a heavier yarn could make the garment more dense and rigid and therefore detract from these qualities. Conversely, if you are knitting a jacket that is intended to be quite structured and cloth-like, going with a slightly heavier yarn (and therefore making the fabric even denser) could work nicely, whereas going with a thinner yarn might not.
Of course, yarn weight is only one aspect of what determines the characteristics of a knitted-up fabric. Considering the fibre content is also crucial. In particular, it is worth mentioning that there are yarns which play by their own rules as far as what weight they are according to their meterage, vs what weight they are according to their 'behaviour.' But that is a separate topic.
And of course, if you are comfortable recalculating gauge and stitch counts, that gives you a lot more freedom to adapt a pattern to any yarn weight you like. But again, that is a topic for another time.
In the here and now, I hope my Handy Chart of Yarn Weights and its accompanying explanation, proves more useful than confusing, and helps you take the first step toward finding a suitable yarn for your pattern.