WPI Yarn Weight Calculator

The WPI yarn weight calculator identifies an unlabeled yarn by its wraps per inch. Wrap the yarn around a ruler, count the wraps in one inch, enter the number, and it tells you the yarn weight category along with a suggested hook size.

Find yarn weight from wraps per inch

Wraps per inch (WPI)

Yarn weight–

CYC number–

Suggested hook–

Wrap snugly without stretching, and average a few counts. Swatch to confirm gauge.

How the WPI yarn weight calculator works

WPI stands for wraps per inch. You wind the yarn snugly, without stretching or overlapping, around a ruler or pencil and count how many strands fill one inch. Thinner yarns fit more wraps into that inch, so a high WPI means a fine yarn and a low WPI means a thick one. The WPI yarn weight calculator maps your count to the standard weight categories.

The Craft Yarn Council groups yarn into eight weights, from lace at the fine end to jumbo at the thick end. Lace yarns wrap around 30 to 40 times per inch, while a chunky jumbo yarn manages only a handful. This tool reads your WPI against those ranges and returns the matching category and a hook size to try.

WPI yarn weight calculator chart showing typical wraps per inch for each yarn weight category

Worked example

Suppose you wrap a mystery yarn and count 10 wraps per inch. That falls in the 9 to 11 range, which the calculator identifies as medium or worsted weight, a Craft Yarn Council number 4.

For a worsted-weight yarn, a good starting hook is roughly 5.5 to 6.5 millimeters, or a US I-9 to K-10.5. From there you would swatch to confirm your gauge before starting the project, since a quick swatch confirms the hook choice and prevents surprises once you are well into the work.

Using the WPI yarn weight calculator on mystery yarn

WPI is the most reliable way to identify yarn that has lost its label, which is common with stash finds and thrifted skeins. Wrap several times along the strand and average your counts, since yarn thickness can vary slightly. The Craft Yarn Council WPI guide shows the exact wrapping technique.

Remember that WPI places a yarn in a category but is not a substitute for a gauge swatch, especially for garments. Once you have the weight, look up the suggested gauge and hook in the Standard Yarn Weight System, then swatch to fine-tune. Very fuzzy or textured yarns can read a little off, so treat the result as a strong starting point.

Frequently asked questions

What is WPI in yarn?

WPI, or wraps per inch, is the number of times a yarn wraps snugly around a ruler within one inch. It is a quick way to gauge yarn thickness and identify the weight category of an unlabeled yarn.

How do I measure wraps per inch?

Wrap the yarn around a ruler or a pencil marked at one inch, keeping the strands touching but not overlapping or stretched. Count the wraps that fill one inch, and average a few readings for accuracy.

What WPI is worsted-weight yarn?

Worsted, a Craft Yarn Council number 4, typically measures about 9 to 11 wraps per inch. Lace yarn sits around 30 or more, while bulky yarns fall near 7 and super bulky near 5.

Why identify yarn by WPI?

Because labels get lost. WPI lets you sort thrifted, gifted, or leftover yarn into the right weight so you can match it to a pattern and choose a suitable hook or needle without the ball band.

Is WPI exact?

It is a reliable guide, not an absolute. Textured, fuzzy, or loosely spun yarns can read slightly high or low, so use WPI to find the category and then swatch to confirm your gauge before starting.