Yarn Size And Math

I recently had to get a lot more technical in my understanding of yarn sizes. I've been calling them things like "that really thin one" or "the fluffy fat one". This doesn't help me to know what the finished cloth will be like when I start plying threads in the warp. Since I ply my weft threads to make fairly thick yarns, I have been unable to reliably repeat the cloth weight, either. I get it close, but there can be a lot of variation from one garment to the next. That's part of why I always weave a garment with its matching parts all at once.

Even before I'm working with the threads in the studio, I have to order them through the mail. It's nice to know just what I'll be getting and how far a 10 lb cone of 16/2 cotton will go, for example.

Before I start with the technical stuff, let me say that I'm not at all sure of any of this. It's what I've pieced together from the little information I could find on the internet layered with my own mathematical ideas. If anyone knows better or could point me to good books or other resources on the topic, please let me know.

Here's what I've learned: there's a formula for worsted wool and cotton that lets you know how many yards of thread are in a pound. You just have to know two magic numbers. For wool, the number is 560. For cotton, it's 840.

Here's how it works. For a single strand of size 1 yarn in worsted wool (called 1s, pronounced ones), there are 560 yards per pound. A thread half that size is called 2s. Half the size means twice as long for the same weight, 1120 yards per pound. If there are two strands of it plied together, it's called 2s2 (twos two) and written 2/2. Each strand is half of a size 1 yarn, but there are two of them, giving 560 yards per pound again, just like the original 1s.

This is where the labeling scheme came from: Strand Size / # of Strands. To convert to yards per pound, multiply the fiber multiple by the strand size and divide by the number of strands.

8/2 Cotton = (840 x 8) / 2 = 3360 ypp
16/4 Worsted = (560 x 16) / 4 = 2240 ypp

This is pretty straightforward. It gets a little more complicated when combining threads and trying to calculate the size of the resulting yarn. I've started calling threads by their "size" instead of their factory name. The stuff that follows is my own collection of ideas. I might be totally wrong, but it seems to work...

If I don't care how many individual strands are in the thread, an 8/2 thread is the same as a 4s thread so I call it "size 4", meaning 1/4 as thick as "size 1". Two of them is the same size as a 2s, which I call "size 2", 1/2 as thick as "size 1".

Now, let's look at a complex weft combination with 5 threads: 2 @ 8/2 plus 3 @ 24/2. I can divide each of the top numbers by the number of strands to get a single yarn equivalent. This gives me...
4/2 plus 8/2,
size 2 plus size 4,
1/2 of a size 1 plus 1/4 of a size 1,
3/4 of a size 1,
which I call "size 1.33"

That last step is tricky. If size 2 (2/1) is half of size 1 (1/2), then we flip the fraction to translate between them. This means that 3/4 of a size 1 is called size 4/3 or size 1.33.

At last, I have a standard way to assign a size to my weft yarns. When I make a sample blanket, I can now track the yarn size along with the color. After it's been wet finished, I'll see how the different sizes behave and be able to produce a dependable cloth of just the right weight.

The two photos below are lit from behind by the sun to give an idea how much light comes through each cloth, and therefore how thick it is. It's really tough to convey weight and texture through images! The dark one is much thicker than the one that's letting light pass through.