This seems like a daunting task, but it's pretty simple. I do a quick bit of comparing three things when I'm helping a customer choose the right yarn for a pattern:
- meterage per gram of suggested yarn
- stitches per 10cm gauge of pattern
- fibre content of suggested yarn
The first thing I do is ignore what the designer calls their yarn's gauge by name. I find names like Sport Weight or DK, etc., are very cultural and sometimes not even close to what we consider those names to mean.
The second thing I do is look up the suggested yarn on Ravelry's website. I like Ravelry's database as it puts all yarns on equal footing by having the manufacturer fill in standard measurements and percentages. I look at how many meters of yarn per 100 grams or 50 grams, as those are the weights in which our yarns come. So if a suggested yarn is 400 meters per 100 grams I just look at our meterage and suggest yarns like Prosper Sock as an exact meter match at 400 meters, or even Chuffed which is 366 meters. So comparing apples to apples or meter to meter is the first place I start.
Then I look at the gauge of the recommended yarn as it's knitted for the pattern and compare that to the yarn I chose based on meterage. If it's within range of what we recommend on our labels I then consider the fibre content (which also speaks to the shape and drape of the garment).
Fibre content is the most nuanced of the three metrics. To play it safe, I'd recommend sticking with similar fibres; wool to wool, cotton to cotton etc. However as you become more experienced with different fibres like silk/wool, and linen/wool blends you'll get a sense for their personalities and will be able to begin to substitute fibres. In the picture above, the yarn suggested is a 100% merino but we substituted a yarn with 10% linen blended with 90% merino. The little bit of linen keeps the lace blocked open nicely and add more drape...and talking fibre is enough content for another blog entry!
Live long and knit!