There are many articles, YouTube videos, and blog posts about sewing leather, but there are a few about one subject many people find confusing: thread size. When originally writing the content for this topic, even I found it to be a bit difficult, so I thoroughly revised my first version and tried to simplify everything as much as possible. The fine people at the Leathercrafter’s Journal liked it so much that they posted it in their January 2024 issue under “So Many Thread Sizes.”
While leatherworkers often think of thread in terms of thickness, thread manufacturers often label thread in terms of weight and length. Manufacturers also use different measurement systems, which causes even more confusion for buyers. Thread size is a frequent discussion topic when I’m teaching classes and helping customers, so hopefully, this article will help you figure it out.
Two Measurement Systems
Many people don’t realize there are two general thread size systems: fixed-weight and fixed-length. The numbering for these two systems progresses in opposite directions; one goes up with thickness while the other goes down. Knowing which one your thread source uses will prevent mistakes.
Fixed Weight Thread Measuring System
In the fixed weight system, the weight of the thread is constant while the length varies. Knitting yarn, cotton, linen, and other organic fibers use the fixed weight system. As their name implies, fixed-weight thread spools are sold at a constant weight. The linen thread we carry on our site is offered in 50-gram spools.
The size number on a fixed spool indicates the length of thread on a spool compared to other spools of the same (fixed) weight. As these spools’ size numbers go up, the lengths increase. Size 332 is the shortest, while 832 is the longest. Sidebar: the ‘3’ in 332 and the ‘8’ in 832 conveys the length. The other two numbers, i.e., the ‘32’ in ‘832.’ indicate plies and yarns, which we will review later.
Fixed weight measurements come from traditional measurements in selling bundles of thread or yarn. The photo below depicts a traditional fixed weight, a hank of wool. One hank is one pound of wool, measuring 560 yards.
Organic threads, like cotton, linen, and wool, have traditional fixed-weight measurements. Thread manufacturers vary their base weights, but they all work similarly. For example, although size 632 linen is similar to the thickness of 28/4 polycotton, each has different fixed-weight bases.
If the weight of each spool from the same product line is constant, but the length increases, then the thread thickness must decrease. The short 332 linen spools have the thickest thread, while the long 832 spools have the thinnest thread.
Let’s look at the label for one type of linen thread:
The ‘50GRS’ stands for 50 grams. While this spool has a specified length, the primary measurement is mass. We must look at both the top and bottom labels to understand the thickness and length.
The 832 number on the bottom of the spool indicates the relative number of lengths (eight) in this spool. We know that the total spool weight is 50 grams, and since this thickness yields eight relative spans, the thread is thinner than a spool of 332 from the same company.
Fixed Length Thread Measuring System
The other primary thread size system is fixed length. Synthetic threads, like nylon and polyester, commonly use fixed-length measurements. They employ more uniform fixed-length measuring units, including TEX, DTEX, and denier.
Tex is a standard measuring unit that I often see in the US. Tex numbers indicate how many grams are in a kilometer of a given thread. One kilometer of 78 tex polyester thread weighs 78 grams. Dtex is another commonly supplied measuring unit. It is similar to tex but indicates how many grams are in ten kilometers of thread.
While you can’t derive the actual thickness from tex, the higher the number, the heavier (and therefore the thicker) the threads become. A 139 tex thread is thicker than a 67 thread. This progression is the opposite of fixed-weight measurements and is where the mix-up starts for leatherworkers when choosing the right size thread for their project.
Even though these spools use the fixed-length system, like fixed-weight spools, they too are often sold in different lengths! You will often see spools of different lengths but the same weight sold. For example, you can buy 1-pound nylon spools of 78 or 139-tex threads; their total lengths will vary since they have different thicknesses.
A ticket number is a commonly found size unit that is a manufacturer’s reference number. It is usually based on the fixed-length system and so is a sub-variant. There is conflicting (and sometimes inaccurate) information out there about ticket numbers. From what I’ve seen, ticket numbers are determined by the actual yield of the thread. For example, a number 69 nylon thread is 70 tex. This offset might be because of how much twist is put into the thread or because varying ply combinations result in slightly different final weights. To me, the ticket-number system is the most quirky type of system, but surprisingly, it seems to be the most popular. I often see sites offering spools like #69 nylon or #138 polyester, which use this system. You can tell these are ticket numbers because the seller also supplies tex or dtex size, which differs from the number in the spool’s name.
Diameter is the measurement most leatherworkers want, but thread manufacturers seldom include it on the label. On our site, we provide thread thickness in millimeters because it is easier for leatherworkers to compare thread types like linen and polyester.
Diameter is a chief aesthetic consideration and is a relative indicator of strength. As thickness increases, strength increases with threads from the same maker. However, the same diameter thread from different manufacturers and different materials are not necessarily equal.
Tying It All Together
I know this was a lot of detailed information, but I hope I’ve clarified the differences in thread sizing and specifications. The main points to remember are:
- Thread labels indicate relative thicknesses.
- Threads like linen usually use fixed-weight measuring. That is, the number goes up, and the thread gets thinner.
- Threads like polyester use fixed-length. That is, the number goes up, and the thread gets thicker.
- If a spool is sold by weight, the thinner thread spools will be longer than the thicker ones.
Now that you know more about how a thread is labeled, you’re ready to dive into the topics in my next post about stitch length, tension, and other questions about using and choosing the right thread for your project.
Note: This post was originally one part of a series of articles on threads for leatherworking. It was published in our newsletter for just our subscribers. If you want to learn more about threads, join our newsletter and get all of the articles in this series. Visit www.fineleatherworking.com/thread-guide to get the entire series.