I’m looking at some wool yarn on eBay. They give the weight of the yarn as 12 wraps per inch. I’ve asked the owner if they know how many yards per pound it is, but they don’t know. Can you point me in the right direction?
It is always hard to know for sure about a yarn that you don't hold in your hand (especially wool, which can vary so much in type, spin, and ply), but you can make some guesses. You'll have to hope that they are wrapping the ruler so that the strands are next to each other without leaving spaces but also without compression. If this is the case, you can conclude that a yarn with 12 wraps per inch would be sett at 6 ends per inch for a balanced plain weave (in an inch of weaving, the same amount of space is taken up by the weft as by the warp, so 6 + 6 = 12). If you know that this yarn is wool, you can consult the Master Yarn Chart. For each yarn, the middle number in a group of three numbers indicates the suitable sett for a balanced plain weave in that yarn. So you can check out the yd/lb for wool yarns that recommend a plain weave sett of 6 ends per inch. When I do that, I see a range of yd/lb of about 400 to 675. Because it's wool, though, the relative thickness/loftiness/qualities of the yarns shown at that sett vary tremendously. So I'd ask them to describe the yarn. Is it a strong yarn with many plies (such as the Paternayan rug yarn) or is it a lofty singles? You should be able to find out enough information to know whether or not it serves your purposes.