Have you ever seen “WPI” and wondered what it meant? It might have other definitions, but in the yarn world it means “Wraps Per Inch”. It might also be handy to know that an inch is 2.54cm.
This is an excellent way to measure your yarn weight. “Yarn weight” means how thick your yarn is, not how much it weighs (confusing, right!?).
How much your yarn actually weighs (in grams or ounces) is dependent on the material it is made of. Acrylic yarn doesn’t weigh much, but cotton can get quick heavy!
Whether it is made of cotton or acrylic (or sheep’s wool, silk, linen, or a blend, etc.), the thickness of the yarn can be labeled by a number or the associated title.
It is common, in some parts of the world, to refer to “ply” in different ways as well.
“Ply” can refer to weight but more accurately defines how many threads are used to create this strand of yarn.
For example, if you spin some sheep’s wool you’ll get a very thin thread. You can then twist two of these threads together and you’ll have 2-ply yarn! Twist 4 of them to get 4-ply, etc. But what if the original thread is thicker or thinner? This is why you can’t necessarily use “ply” to determine how thick the yarn is.
Here in Canada our yarn labels have a lot of handy information.
The number on the tiny yarn skein tells you the yarn weight. Craft Yarn Councils has a standardized chart:
The next 2 symbols on the photo above are for gauge swatches. Yarn labels will usually give a gauge swatch for knitting and crochet. The most common size for a gauge swatch is 10 x 10 cm / 4 x 4 inches. Using the hook size listed, created a square of single crochets and measure how many stitches wide by how many rows tall and that will tell you if you crochet tighter or looser than the label expects you to do. This label above says they get 13 single crochets wide by 14 rows high in their 4″ gauge swatch.
You might have another ball with a “Medium – 4” label but the gauge swatch is slightly different. This is because there can be quite a wide range of thicknesses in each category.
Wraps Per Inch
But what about yarn where you’ve lost the label? Or it never had one to begin with (hello, mill ends)? We can do a Wraps Per Inch (WPI) test to figure out our yarn weight.
I created a quick video:
You can now use this handy information to help you choose what hook size to use for your yarn!
The craft yarn council’s wraps per inch chart:
And, after you have your WPI number (and it’s associated yarn weight from the first chart), go to the next chart and look for the recommended hook size for that category of yarn:
You can think of this as a starting place because you may still need to adjust your hook size.
If your swatch is too stiff, go up a hook size (meaning, go up 0.5 mm).
If it is too loose, go down a hook size.
If you’re trying to match a designer’s gauge swatch and they say you should have 16 stitches in a 4 inch area, but you keep getting too many stitches then you should try going up a hook size. If you aren’t getting enough stitches (for example, you only get 14 stitches but they say you should get 16) then try going down a hook size.
If it’s the height of your stitch that’s causing you problems then it’s a tension issue.
Ultimately, matching gauge is usually not critical in my patterns. Blankets are very forgiving that way. A sweater is less forgiving.
And besides changing the finished size of your project, you could be using more or less yarn than expected (which could cause problems).
I hope this helps!
Oh, and I do have a really old video on creating a gauge swatch for the interlocking crochet projects: https://youtu.be/BLgCb9VaPMU
There’s definitely a lot more to be said about yarn and yarn weights but this was maybe a good place to start your learning journey?