Yarn thickness is measured in Wraps Per Inch (WPI) to determine its weight category. This method of measurement can be used on any yarn, from those mystery skeins lurking in your stash to handspun yarns and even commercially made yarns that you suspect might be mislabeled (it happens more often than you’d think!). Whether you’re looking to substitute in a yarn for the pattern of your choosing, or you’re trying to use up leftover bits in a scrappy knit or crochet project, taking the time to measure WPI can ensure consistency in your yarn choices, resulting in a project you can take pride in!
While there are many tutorials out there showing how to measure WPI using everything from a ruler to a pencil, we’ve created our WPI Toolkit to ensure the utmost accuracy. Other methods can distort and untwist yarn during the wrapping process, which gives you an inaccurate measurement. Our handy tool is designed to gently rotate to wrap the yarn around it, preserving the plumpness of the yarn in the process. Simply count the number of wraps in the one-inch marking to determine the WPI:
- Lace weight yarns: 35+ WPI
- Fingering weight yarns: 19-22 WPI
- Sport weight yarns: 15-18 WPI
- DK weight yarns: 12-14 WPI
- Worsted weight yarns: 9-11 WPI
- Heavy worsted weight yarns: 7-8 WPI
- Bulky weight yarns: 6 or fewer WPI
Smooth yarns are easy to measure, but “hairy” yarns such as mohair or eyelash yarns are a little more tricky: we recommend wrapping these yarns over a larger section of the tool (additional markings are included for this express purpose!) and then taking an average to determine the final WPI. For example: here we’re measuring a mystery mohair yarn and have wrapped it around 3 one-inch markings:
The total number of yarn wraps is 108. To get the average number of wraps for this yarn, simply divide it by the number of one-inch markings you wrapped - in this case, 3.
108 divided by 3 = 36 WPI
According to the chart above (which is also found on the Knit-Kard included in our WPI Toolkit), this is a lace weight yarn.
Our Knit-Kard also recommends knitting needle and crochet hook sizes for each yarn weight, although you may have a personal preference, or your chosen pattern may specify a certain size to obtain the stated gauge. For this reason - especially if you are substituting in a yarn - we recommend knitting or crocheting a gauge swatch before you start your project. While yarns with the same thickness (WPI) can be exchanged, you will want to make sure that the fiber content has the same properties (for example, drape or stitch definition) and that you are meeting the required gauge with your needles or hook.
We hope this post gives you a better understanding of the Wraps Per Inch (WPI) measurement of yarn. If you liked this post, pin it!