Since I knit a lot of socks, I'm always looking for new wool to try. I find it hard, even in a big city like Vancouver, to find decent sock wool, so I do a lot of online shopping. I have learned to read those "yardage" labels to get a better feel for how yarn will knit up. Here's what I've learned.
Any yarn purveyor worth your while should be able to tell you the weight per unit length of their yarn. For factory-made commercial yarns it'll be on the label in units like yd/oz or m/g or sometimes, yd/g. For smaller yarn producers from the internet, they sometimes give you the numbers in yards or m per ball (2 oz or 50g) or hanks or skeins (4 oz or 100g).
Given how you like to knit, set some limits as to what yarns will and won't work for your style and preferred equipment. Start with your favourite yarn, figure out it's "yardage" in m/g, and then use this as a benchmark for yarn shopping.
For example, I prefer to knit on 2mm needles, and I love Scheepjes Invicta. This wool is labelled "210m / 50g", which comes out to 4.2m/g. I also know that Kroy sock wool, which is really at the limit of what I like to knit on 2mm's, is 152m/50g, which works out to 3.04m/g. So this is my "hard limit" for sock wool.
Now, I also have to be able to figure out these limits in "American" units, which are yd/oz or sometimes yd/g. I find, thanks to many unit converters on the interwebs, that
1 yd = 0.9m, and
1oz = 28.3 g, and so
31 yd/oz = 1 m/g
So, my "hard limit" works out to be:
3 m/g = 3.33 yd/g = 167 yd/50g (ball) or 333 yd/100 g (skein)
3 m/g = 93 yd/oz = 186 yd/2 oz (ball) or 372 yd/4 oz (skein)
anything that doesn't have at least this yardage is not going to work for me on 2mm needles. For thin, everyday socks, I won't use anything that comes in at less than 400 yd/100 g.
I've figured out:
Knitpicks Risata: 3.6 m/g (with elastic)
Knitpicks Stroll: 4.2 m/g (but not tightly plied, so "squooshy" and knits a bit fatter and floppier)
Lang Jawoll (3 ply) : 4.2 m/g (tight ply)
Scheepjes Invicta: 4.2 m/g (tight ply)
Malabrigo Sock: 4.0 m/g (tight ply)
Regia Stretch Color : 3.9 m/g (with elastic)
These are yarns I've tried and work well for me.
With experience, I'm starting to get a better feel for the effect of plying as well. This isn't on the label and you have to actually hold the product in your hand to see it. Plying describes how tightly the wool is twisted during spinning. Knitpicks Stroll line is not very tightly plied, and so it is "soft" or "squooshy". This makes the wool less springy, less elastic, than some more tightly plied products such as Invicta or Malabrigo. I'm not sure it affects the wearing characteristics, but it sure has an effect on how "floppy" the sock feels. Since I don't like baggy or slouchy socks, I tend to prefer more tightly plied yarns.
I've tried yarns with elastic in them - KnitPicks Risata and Regia Stretch are two examples - and they hold their shape beautifully. These yarns can be "hard", and almost artificial-feeling though, especially the Regia. But I've found they have wonderful stitch definition (beautiful for cables) because they have no "halo" - they are not fuzzy. And they wear like iron! They're indestructible and they keep looking good, wash cycle after wash cycle.