What is WPI and Why is it So Awesome?

Water Polo Iguanas? Wonderful Pie Initiative? Wishing Penny Inscription? What exactly does WPI mean? You may have seen that acronym on a yarn label recently, along with a number. Here's the deal: WPI stands for Wraps Per Inch. Wraps per inch? What does that mean? Well my friends, as it turns out it means a lot when it comes to comparing yarns and achieving proper gauge in your crochet projects. Are you ready for a little lesson on WPI? Allow me to blow your mind and change the way you look at yarn.

WPI is an essential piece of information when it comes to yarn substitution. Say you want to crochet a hat that calls for 200 yards of a particular worsted weight yarn, Vanna's Choice. You don't have any Vanna's in your stash right now but you do have 200 yards of Cascade 220, a different worsted weight yarn. You can just use the Cascade instead, right? I mean, they're both worsted weight... Unfortunately, the answer is "not necessarily". Just like two differently-branded hooks of the same size won't necessarily make the same sized stitches, two yarns of the same weight aren't necessarily the same. Take a look at the yarns in question. They are both worsted weight and yet they are not the same thickness.

 
What?! I know. It's terrible and frustrating. However, WPI is here to save the day. It's the great equalizer. Here's how to find the WPI of any yarn. Take a ruler and gently wrap the yarn around and around it. Push the yarn strands together into an inch and count how many strands there are. That's all there is to it! Here's Berroco Comfort Worsted. It has a WPI of 9.

The higher the WPI the thinner the yarn because more yarn strands can fit into an inch. In order to get an accurate measurement be sure you aren't yanking on your yarn and wrapping it too tightly as seen below.

So, according to WPI measurements, if I wanted to use Berroco Comfort in place of Cascade 220 I wouldn't encounter a problem. However, if I wanted to use Berroco Comfort or Cascade 220 in place of Vanna's Choice I might have some issues since the former is actually thinner than the latter, even though they are labeled with the same weight. This issue becomes even more pronounced the larger the yarn. I cannot emphasize enough how important it is to check the WPI of a yarn that is labeled bulky or super bulky if you plan on subbing in something else. Some bulky yarns are closer to worsted in thickness!

Are you a little nervous about obtaining an accurate WPI measurement for your yarn by yourself with your ruler? You're in luck! Ravelry has got you covered and almost always lists the WPI of yarns in its yarn database.

As a final note, it's just fine to sub in a yarn that doesn't have the same WPI as the original yarn. Just be aware that you may now have to change your hook size in order to obtain the same gauge and finished item size as the designer's sample.

So there you have it! WPI measurements can save you a lot of trouble when using one yarn in place of another, especially when the finished size of the item really matters. Have questions about WPI? Ask away in the comments!