Wraps Per Inch Explained

The method

1. Find something straight with a known inch. A ruler, a pencil marked at an inch, or a purpose-made WPI gauge.
2. Wrap the yarn around it in a single layer, each wrap touching its neighbor.
3. Do not pull it tight. Compressing a lofty yarn makes it read finer than it is - this is the single biggest source of error.
4. Do not leave gaps either. Touching, not squashed.
5. Count the wraps across one inch. Measure in the middle of the wrapped section, not at the end.
6. Do it three times in different places along the yarn and take the middle figure.

Why we do not publish a WPI-to-weight table

You will find WPI tables everywhere, and they do not agree with each other. The reason is structural rather than sloppy.

• The Craft Yarn Council standard does not define WPI. Its weight system is built on knitted gauge over 4 inches, not on yarn diameter.
• Loft varies enormously at the same diameter. A woolen-spun single and a firmly plied yarn can wrap identically and knit completely differently.
• How hard you wrap changes the answer, and there is no standard wrapping tension.
• The bands overlap in every published version, which means any given number sits in two categories.

We are not going to print a table we cannot source to a standards body, because the whole point of this site is not inventing numbers. What we can give you is a better method.

The comparison method, which is reliable

Instead of measuring your mystery yarn and looking the number up, measure it and measure a yarn you already own and have a ball band for. Compare the two numbers directly.

1. Take a yarn with a known weight category from your own stash - a labeled worsted, DK or fingering.
2. Wrap both on the same ruler, at the same tension, on the same day.
3. Compare. If the mystery yarn wraps the same number of times as your known worsted, it is a worsted-ish yarn.
4. Confirm with a swatch. WPI narrows it to one or two categories; gauge settles it.

This removes every problem with published tables at once, because you are measuring both yarns with the same ruler, the same hands and the same tension. It is the version of the method that actually works.

Reference yarns to keep for this

Keep a short length of two or three of these taped to a card. It turns every future mystery skein into a two-minute job. The full weight bands are on the yarn weight chart.

What WPI cannot tell you

• Fiber content. A burn test can indicate it; wrapping cannot.
• Yardage. Weigh the ball and compare against a known yards-per-gram, or measure a length and weigh that.
• How it will knit. Loft, ply and twist all change the fabric at the same diameter.
• Whether it is superwash. Nothing visible will tell you this, and it changes the care label completely.

For the rest of the identification job - yardage by weight, and what the fiber is likely to be - see yarn substitution and how to read a yarn label.

Working out yardage without a label

1. Measure a known length - a yard, or a meter - and cut it.
2. Weigh it on a kitchen scale that reads to a gram, or to 0.1g if you have one.
3. Weigh the whole ball.
4. Divide. Ball weight divided by sample weight, times the sample length, gives total yardage.
5. Round down, and buy a margin.

That arithmetic is exact, unlike the WPI lookup, and it is what you actually need before starting a project. How much yarn do I need takes it from there.

Then swatch

Wraps per inch narrows an unknown yarn to a category or two. A swatch tells you what it does on your needles in your hands, which is the number a pattern actually cares about.

Wrap first because it takes two minutes and tells you roughly which needle to reach for. Swatch second because it is the only thing that settles it - knitting gauge and swatching has the method.