A Cheater’s Guide to WPI (Wraps Per Inch)
What is “WPI”? This abbreviation, which stands for “wraps per inch,” is a handy way of determining yarn weight for a yarn with a missing ball band. It’s especially handy for determining the weight of hand-spun yarn.
The downside of using WPI to figure out your yarn weight, of course, is that you have to have a chart handy. Is 11 WPI DK or worsted? Well, it really depends on the chart — there are several variants. Fortunately, there are two easy (and quick!) ways to determine the weight of your yarn, whether it’s mystery stash or hand-spun. They’re not perfect, of course, and you might still have to swatch, but there’s no chart needed.
Cheater method #1: Use a needle gauge
Grab a needle gauge, fold a length of your weight-less yarn in half, and poke it through some of the holes. You want the yarn to be snug in the hole so it’s not flopping around and falling out, but you want a little wiggle room too — not so tight that you have to really force it in there. When you find a hole that seems comfortable, start your swatch (or if you’re swatch-averse like me, your project) with the needle that the hole corresponds to.
This method won’t tell you the actual weight of the yarn, so it’s a little harder to determine whether your yarn will work as a substitution for a given pattern, but I think the ease and speed makes up for that. And if you’re making something pattern-free, like a simple hat or a scarf, this method will show you a comfortable needle for your yarn.
One more note: This method will give you a needle size that is closest to the typical needle suggestions on a ball band. The fabric it produces (depending on how tightly or loosely you knit) will be a typical, relatively firm fabric. If you know you’re going to want a looser fabric, go with a hole size where the yarn is a little looser. If you know you’re going to want a firmer fabric, go with a hole size where the yarn is a little tighter.
Cheater method #2: Eyeball it
If you’ve been knitting long enough with a wide variety of yarns, you can probably just look at your yarn, compare it to your mental database of yarns you’ve worked with, and make a reasonable guess at the weight. I could probably spot a fingering weight yarn at 20 paces, but not everyone has that experience. If you don’t, that’s okay. Just grab some yarn that seems comparable and lay some strands out in front of you.
You can compare, side by side, various weights of yarn and see where yours comes closest. Above we have, from left to right, some hand-spun Romney, a fingering weight (Brown Sheep Cotton Fine), a sport weight yarn (Malabrigo Arroyo), a DK weight yarn (Fleece Artist Merino 3/6), and a worsted weight yarn (Verdant Gryphon Mondegreen). Pick up your mystery yarn, in this case, the handspun Romney, and place it next to each known yarn in turn. In the example above, you can see that it comes closest to the third yarn from the left, the sport weight. I can reasonably conclude, then, that my hand-spun is probably a sport weight. If you want to be more sure of your choice, compare the mystery yarn to several different kinds of yarn in the weight you think comes close. Yarn weights vary, even within a category, so using multiple examples for comparison will give you a better idea of the weight if your yarn seems to fall between two categories.
According to the chart used in our post on determining yarn weights, my yarn would be classified as a DK weight, but at 14 WPI, a slightly different chart might give me a different answer, since my count is sort of between weight classes. You could probably use a yarn that falls between categories like this for patterns calling for either weight, you would just have to swatch.
All that being said, I still find WPI to be a useful tool. If you pick a chart and stick with it, you can classify all your hand-spun consistently. There’s something to be said for knowing fairly definitively what weight class your yarn falls into. All three methods can be useful in different circumstances, and with these two “cheater’s methods,” you should have no trouble matching yarn to needles to pattern.