"Another option, which you may find more frustrating, is to take the yarn weight, gauge, and needle size from the pattern, and try to find something that is similar in your stash and swatch."
This is kind of what I have to do by default since the yarn weight is measured in a very different way in my country, so when Im reading patterns from other countries (which means... almost always!) I more or less have an idea of what "size" yarn Ill have to use. But yeah, sometimes it is frustrating to find something similar enough, specially when people describe it as something like "Oh yes this uses a worsted weight... but more on the thin side". Then all my knowledge on converting worsted weight yarn goes out the window and I have to find new matches ;P
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If I am working with the same material, I find meters per gram to be a good measure for how well a yarn will work in the place of another. Though I usually dont care much... as long as the needle os right, the only thing that changes is the size of the final item, and for most of my amigurumi or motifs a smaller or bigger version is ok.This only really matters for wearables, and in that case I am making a test swatch anyway (there were too many cases of miss fitting items already :( )
chekccard