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wraps per inch method of yarn identification

the wraps per inch method | jakigu.com | green yarn wrapped around a metal ruler

What are wraps per inch?

Wraps per inch is a simple method of measuring yarn thickness and determining yarn weight. It can be equally helpful to crocheters, knitters, weavers, or spinner. Professionals often refer to it simply as wpi.

How do I measure wraps per inch?

In its essence, the wraps per inch method couldn’t be simpler. First, you wrap yarn around a stiff object for a length of 1 inch. Then, you count how many wraps there are within this length.

tools to determine wraps per inch | jakigu.com | tape measure, ruler, pencil, and yarn

Tools

Unsurprisingly, there is a variety of specialized tools for counting wraps per inch. I think some of them look really cool, but they are all truly unnecessary. All you need to measure wraps per inch is a ruler. Or a tape measure, and perhaps a pencil. I’m all for doing more with less.

Methods

1. Using a ruler

This is the simplest, and my favorite, way of counting wraps per inch. It requires nothing but a ruler, and it works with any yarn.

Wrap yarn around a ruler for a length of 1 inch. Make sure to lay the loops as close to each other as possible but do not overlap or cross them. Then, count how many loops there are within this 1-inch length. That number is your wraps per inch number, or wpi.

wpi, the ruler method | jakigu.com | teal yarn wrapped around a metal ruler

2. Using a pencil and a ruler

Use this method if wrapping yarn around your ruler could potentially damage or stain the yarn.

Wrap yarn around a pencil (or a dowel) for a length of 1 inch. Place the pencil against a ruler (or a tape measure), and count the number of loops within a 1-inch length to get your wpi number.

wpi, the pencil and ruler method | jakigu.com | pink yarn wrapped around a brown pencil next to metal ruler

3. Using a tape measure at the store

Sometimes, you will want to know the wpi number of yarn before you buy it. Use this simple method to determine the wpi of yarn without disturbing the hold of the skein at the store. I like to carry a tape measure in my purse for this purpose, but you can also use a ruler.

Find a spot on the skein where the yarn strands lay most neatly next to each other for the width of at least 1 inch. Place your tape measure perpendicularly across the yarn strands. Then count the number of strands within this 1-inch space to get your wpi number.

wpi: the tape measure method | jakigu.com | white tape measure held with two fingers on top of a skein of yellow yarn

In what situations can I use the wraps per inch method?

Generally, the wraps per inch method is used for two main purposes. It can help you identify suitable yarn alternatives based on yarn thickness, and it can help you determine yarn weight of any yarn.

At one point or another, we all will face the need to substitute the yarn in a pattern with another material. Whether we are unable to source the suggested material, or the suggested yarn is too expensive or it doesn’t come in colorways we like, sometimes it just might not work for us.

Wraps per inch allows you to compare yarn across brands based on yarn thickness. Knowing yarn thickness in turn helps you choose the most appropriate alternative to the yarn called for in a pattern if you do not or cannot use the suggested material. Yarn with the same wpi number is the most likely to produce results comparable to using the suggested material.

Alternatively, the wpi number can help you determine yarn weight of any yarn, no matter its origin. Whether you have a few hand-me-down skeins without labels or a couple of unmarked leftover balls, the wraps per inch method can help you quickly gauge this mysterious yarn’s weight. Craft Yarn Council has a handy chart for determining yarn weight based on WPI here.

wraps per inch method tips | jakigu.com | three yellow pencils, each wrapped with white yarn of different thickness, with metal ruler in the foreground

Tips for measuring wraps per inch

Wraps per inch in the metric system.

Metric system users can get the same wpi number by counting the number of yarn loops within the length of 2.5 cm.

Wrap more length for a more accurate wpi number.

For greater accuracy, or when counting wpi of bulkier yarns, wrap the yarn around your measuring tool for a length of 2, or even 3 inches. Count the number of wraps within this whole length. Then, calculate the average of wpi: divide the total number of wraps by the number of inches you wrapped the yarn around. This number will be more accurate

Consider the fiber content.

If you are relying on WPI to find a substitute yarn, keep in mind that different fibers behave differently when worked into a project. Whenever possible, choose substitute yarn with the same WPI and the same fiber content. Acrylic yarn tends to be more squishy and soft. Its strands are often quite stretchy: the tauter they are pulled, the thinner they can become. Cotton, on the other hand, is a stiffer yarn, lacking much of the acrylic’s flexibility. Stretching cotton strand will hardly affect its thickness. Ultimately, an item made with the same hook based on the same pattern might inevitably be slightly larger when worked in cotton than when worked in acrylic, even if the wraps per inch number of both yarns is the same.

No method is perfect.

Even if we are able to use the exact brand of yarn and hook as the pattern we follow, our results may vary. Our personal stitch tension, the way we work our stitches or hold the hook and yarn – they all may affect the look and feel of our finished item. Add to the list of variables a different hook and/or different yarn, and the craft of crochet becomes the art of crochet. Wraps per inch method can help us eliminate at least one variable, but slight variances are inevitable.

Thankfully, many projects are quite forgiving of these minuscule differences. You’ll often find room for some manipulation and adjustment if your crochet piece needs small adjustment. But if you are planning to create something with very specific dimensions, consider working up a gauge swatch before you start.

Gauge swatches are the only sure way to determine whether the yarn and hook you are working with will yield finished items of desired size. Even here bear in mind two important facts. The first is that your individual stitch tension can vary over time. That means each gauge swatch should be considered a time-sensitive sample. If you don’t start your item within a few months of working a swatch, consider making a new one. Additionally, the manner of crochet execution matters. Only work your swatch in back-and-forth rows if your item is also worked in rows. For items crocheted in rounds or spirals, make sure you work your swatch in the like manner.

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