Let’s face it, most knitters and crocheters have a box of unmarked yarn laying around just waiting to be used. How do you know the weight of the yarn? Here is some useful information to help you identify your yarn's weight category number.
Read the label
This may seem obvious, but typically when purchased, most yarn comes with a label that gives you important information about the yarn.
There are also more useful instructions on how to wash your finished product, fiber content, recommended knitting needle size, yarn length, and the color/dye lot number.
This information should also include what size the yarn is, or what “weight,” referring to the thickness of the yarn. Recommended stitches per inch can also typically be found on the label.
The best advice for any knitter or crocheter is to try and keep the label with the yarn. You can wrap your yarn around the label, make your own label with tape, or put the yarn in a marked bag.
Keeping yarn neat and organized is the key to avoiding having mystery odds and ends.
WPI (Wraps Per Inch)
For any random yarn you might have there is another way to identify yarn weight. This method involves counting wraps per inch. You can use a ruler or a pencil. Measuring WPI is very quick and easy!
Simply take the mystery yarn and wrap it around your ruler or pencil so that you have an inch of yarn wraps around your object. Then count the number of times the yarn wraps around said object. So, if your yarn wraps around eight times then the WPI measurement of your yarn is “eight.”
Measuring using WPI will get you close to your yarn’s actual weight, but keep in mind, it can vary depending on how tightly you wrap the yarn around your object. By making a gauge swatch you can more accurately determine how the yarn and needles work together to make your garment.
Here is a chart to help convert WPI into weight classes of yarn as well as other useful information: