Have you ever found a ball of yarn in your stash that no longer has a ball band? Some yarn weights are easy to identify, but others can be a bit trickier. How do you know what weight it is? Is it worsted, light worsted, or something else? This is when we call on a useful yarn hack called wpi, or wraps per inch. Simply put, wpi measures how many wraps of yarn are in 1 inch.
Wraps per inch is a particularly useful technique when using things like t-shirt yarn where the weight isn’t clear, or for those mystery balls from the thrift store!
Using the wraps per inch technique, you can figure out the weight of any yarn. Just follow the easy steps below.
What you need to measure wraps per inch
- Ruler or gauge tool measure
- Something to wrap your yarn around, such as a pencil or piece of dowel*
It is helpful to have a ruler that measures in inches, as this technique is wraps per inch, however, if you only have a metric ruler available, you can use this simple conversion of 1 inch = 2.5cm.
It is best to use something cylindrical and even, such as a pencil, knitting needle or piece of wooden dowel. The thickness is not too important.
How to measure wpi
- Begin by holding your yarn in one hand and the pencil in the other hand
- Carefully wrap the yarn around the pencil, making sure to use an even tension. The wraps should lie snugly side by side. There is no need to pull extra tight, but the wraps do need to fit securely and evenly around the pencil
- Continue wrapping for a few inches
- Once you have finished wrapping, count how many wraps fit into 1 inch. It is a good idea to measure in several places and take an average number of wraps if your measurements differ
- Refer to a chart (such as this one) to determine what yarn weight category your yarn fits into
Top tip: if you frequently find yourself needing to measure your yarn weight using the wpi method, then place a length of dowel with the inches marked off in your project bag.
How does wpi work?
The diameter (or thickness) of your strand of yarn determines which weight class it fits into. Since a heavier weight yarn is thicker, it will require fewer wraps to cover a 1 inch surface than a lighter weight yarn.
It is important to remember that this method is not an exact science. The number of wraps that you get will be influenced by how tightly you have wrapped your yarn. For this reason I’d suggest practicing with some yarn where you know the weight. And always do a gauge swatch!