Unraveling the Needle Size Mystery: What Size Knitting Needle Should I Choose for Alpaca?

I once edited a book that included a hat knitted in bulky baby alpaca yarn. The hat was popular, and we got several calls from knitters asking about an error in the pattern. The designer called for size 7 needles and 5½ stitches per inch, and the yarn label called for a size 11 needle and 3 stitches per inch. Surely we must have made a mistake—the designer couldn’t have knitted the hat on needles so small? But knit she did, and the yarn made a firm but lovely ribbed fabric.

Another time, a designer submitted a swatch using a fluffy discontinued alpaca/silk yarn that the manufacturer called a DK-weight. I assigned a wool yarn described as fingering-weight and was confident that the new gauge would be the same or smaller. But the designer wrote to say that the new yarn was much bulkier than her original swatch; she'd had to go up several needle sizes and change the pattern to omit several repeats. Later I asked the wool yarn’s manufacturer whether she thought the yarn should have been labeled as sportweight, and she shrugged. “I like it on smaller needles and a tighter gauge, so we call it fingering weight,” she explained.