Thread Weight vs. Yarn Weight: A Guide to WPI and Yarn Substitution

There are all kinds of numbering systems to describe and compare yarns. It’s no wonder a crafter might get a little confused. If you find yourself lost between size 10 thread, size 0 yarn, 2-ply yarn, and other numerical descriptions, you’re not alone. Understanding how wraps per inch (or WPI) works is the secret. I wanted to see if I could substitute some size 3 crochet thread I had in my stash for the fingering-weight yarn used in Interweave Crochet Summer 2018 for the Aquatic Swimsuit. So I conducted an experiment. Today, I’m breaking down yarn weight and crochet thread sizes so you can substitute yarns with ease!

What is WPI?

Wraps per inch indicates the thickness of a yarn. If you’re substituting yarns and trying to get gauge, you’ll want to find yarns that have the same number of wraps per inch (WPI).

How do you determine the WPI?

1. Use a WPI tool.

Some WPI tools look like a small wooden dowel with a knob on the end. Others have an inch of space carved out of a rectangle of wood. To use a WPI tool, wrap your yarn around the dowel or wood piece from one inch marking to the next, with each wrap of the yarn touching the previous wrap and no wraps overlapping. Be careful not to pull the yarn too tight. Count the number of times the yarn wraps around the tool within one inch.

yarn weight can be determined using a WPI tool

2. Use a pencil and a tape measure.

You can replicate a WPI tool by wrapping yarn around a pencil (the pencil must have the same circumference for its entire length), with each wrap of the yarn touching the previous wrap and no wraps overlapping. Again, be careful not to pull the yarn too tight. Once you have a few inches of wrapped yarn, hold the pencil up to a tape measure or ruler and count the number of yarn wraps in one inch.

How does WPI measure up to designated yarn weights?

The Craft Yarn Council has established a numbering system for classifying yarn weights. You will often see these numbers on yarn labels or in Interweave patterns. The United Kingdom and Australia might use different terminology (for example, a number 4 medium-worsted-weight yarn in the United States might be referred to as an Aran-weight in the United Kingdom or a 10-ply in Australia). I won’t go into that detail in this article, but you can consult Love Knitting for more information on that terminology.

Below is a list of how WPI correlates to yarn weight numbers.

CYC Weight 0 Lace = 30–40+ WPI
CYC Weight 1 Super Fine = 14–30 WPI
CYC Weight 2 Fine = 12–18 WPI
CYC Weight 3 Light = 11–15 WPI
CYC Weight 4 Medium = 9–12 WPI
CYC Weight 5 Bulky = 6–9 WPI
CYC Weight 6 Super Bulky = 5–6 WPI
CYC Weight 7 Jumbo = 1–4 WPI

How does crochet thread fit into all this?

Crochet thread is a bit different from other yarn because it is typically made of mercerized cotton with an especially tight twist. To read more about the properties of crochet thread, check out “Lovable, Wearable, Cotton: A Spinner Explains the Magic of Crochet Thread.”

If you’re familiar with crochet thread, you probably know that it has its own numbering system. Crochet thread comes in sizes 3 through 100, with 3 being the thickest and 100 being so thin that it resembles sewing thread.

How does thread WPI compare to yarn WPI?

I grabbed some crochet thread that I had around my craft room, and here is how a few crochet threads measure up in WPI.

Size 3 Crochet Thread = Approximately 21 WPI (similar to CYC 1 Super Fine)
Size 5 Crochet Thread = Approximately 23 WPI (similar to CYC 1 Super Fine)
Size 8 Crochet Thread = Approximately 36 WPI (similar to CYC 0 Lace)
Size 10 Crochet Thread = Approximately 38 WPI (similar to CYC 0 Lace)
Size 20 Crochet Thread = Approximately 52 WPI (very, very small, so we’ll stop there)

So now I know, I probably can substitute that size 3 crochet thread for the fingering-weight yarn used in the pattern, but I’ll want to check my gauge, keep my stitches a bit loose, and maybe even consider going up just a tiny bit in my hook size.

If you have any mystery yarn at home, you can use the information in this post to see what weight it is. If you’d like to find out what kind of mystery fiber it might be, check out our post on a fun game called “What’s That Fiber?”

Happy Yarn Substituting!
Editor, Interweave Crochet

There Is Always Something to Learn About Your Craft!

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