How to measure wraps per inch (WPI)

When you finish a skein of hand-spun, the first question is usually: what weight is it? Wraps per inch is the quickest way to find out, and you already own everything you need.

What is WPI?

Wraps per inch (WPI) is the number of times a yarn wraps around a one-inch length without overlapping or crowding. The finer the yarn, the more wraps fit into that inch; the thicker the yarn, the fewer. Because commercial yarn-weight categories (lace, fingering, DK, worsted, and so on) map roughly onto WPI ranges, a quick wrap tells you which category your yarn belongs to.

It is especially useful for two things: identifying an unlabelled ball of yarn, and giving your own hand-spun a recognisable weight so you can match it to a pattern.

How to measure it, step by step

1. Find a ruler with a clear one-inch span, or a dedicated WPI gauge (a small tool with a one-inch notch).
2. Wrap your yarn around it, laying each wrap snugly next to the last. Don't overlap the wraps, don't leave gaps, and don't stretch or squash the yarn. Let it sit at its natural thickness.
3. Count how many wraps fill exactly one inch. That count is your WPI.
4. For accuracy, measure in two or three different places along the yarn and take the average, because hand-spun naturally varies a little.

WPI to yarn weight chart

Charts differ by a wrap or two between sources, so use the nearest band rather than treating the number as exact. If your yarn falls right on a boundary, your pattern's gauge swatch is the final word.

Try our free WPI calculator →

WPI and grist together

WPI tells you how thick a yarn looks; grist tells you how dense it is: the length you get per unit of weight (yards per pound, or metres per gram). Two yarns can share a WPI but have very different grist if one is spun more firmly. Recording both gives you a complete fingerprint of a yarn, so you can spin it again from scratch.