Picking the perfect thread for your project shouldn’t be overwhelming. Most of the time you can just grab a spool of all-purpose polyester thread, fill a bobbin with it and you’re on your way. No problem.
The problem and subsequent overwhelm happens when you grab that spool of thread and it doesn’t work quite like you thought it should. Or, maybe you snagged a cone of serger thread and tried using that in your sewing machine. Or you bought a different brand of serger thread cause you wanted the color IT came in. Or, you just went to buy thread, and all of a sudden you’re faced with gasp Options.
I cover thread types in a different article. This one is more of a deep dive into thread so you can understand the differences between two seemingly same threads (hello thread weight), understand some of the terms used in thread descriptions and, of course, troubleshoot issues that may arise from threads because of these differences listed here.
So, let’s get started.
Thread Manufacturing
Just like in fabric, there are construction types and fiber types and they shouldn’t be confused.
When it comes to fiber preparation, manufacturers start by selecting either natural fibers like cotton or synthetic ones like polyester. In my experience, polyester thread is incredibly versatile and what I reach for most often. It’s stronger than cotton and has a bit of stretch, making it perfect for most everyday sewing projects. But there are times when you want or need a natural fiber. And there may be times you want or need a different synthetic fiber like nylon or kevlar. (What?!? they make Kevlar thread? Yes. Yes, they do.)
The chosen fibers for the thread are then twisted together into some configuration to make the thread. A single strand of twisted fibers is called a Ply. The amount of twist affects the thread’s strength and appearance, which is why some threads look smoother than others.
There are 4 main configurations you’ll find in thread:
- Monofilament
- Spun
- Corespun
- Corded
Monofilament
Monofilament is essentially a single ply of thread. However it is often not actually twisted fibers but more like an extruded strand of thread more like a really long cooked spaghetti noodle. You usually will see nylon or polyester monofilament because these are synthetic and can be made more extruded than twisted unlike cotton or wool.
Spun
You might see “spun” thread which would mean that each ply is comprised of twisted together fibers. But there is no additional core of more tightly spun fibers. I’ve seen serger thread listed as spun thread. It’s still strong, but could be slightly finer than corespun thread.
Corespun
Corespun thread will have a core of tightly twisted fibers in each ply around which more fibers are twisted to make the ply. Then 2 or more of those plys will be twisted together to make thread. The core fiber might not be the same as the sheath of the ply. So, all-purpose polyester wrapped polyester core thread is core spun, but so is all-purpose cotton-wrapped polyester core.
Corded
Corded thread is even more twisting of plys that are then twisted together. so think of a spun thread that is then twisted with another spun thread, and the result of that new thread is then twisted with another like it to make a thread.
Yarn is a good “easy to see with the naked eye” thread stand in. It’s far easier to see the twists of the fibers in yarn. But the same philosophy holds true for thread. Most yarns would be considered spun.
Thread Finishes
After spinning, threads go through various finishing processes. These treatments can help the thread perform better in your machine and make it more durable. Or, the finish might be imparting some special characteristic to the thread for a very specific purpose. Let’s take a look at some terms you may run into.
- Mercerized
- Unmercerized
- Glazed
- Gassed
- Bonded
- Anti-wick
- Fire-Retardant
Mercerized
One of the most common terms you’ll see is “mercerized” especially with cotton threads. This process preshrinks cotton thread, coats the thread to give it more luster, makes it stronger and easier to dye. Strangely, it makes it less absorbent. And fortunately, less prone to mildew.
Unmercerized
Unmercerized thread is left in a more natural state. It’s quite absorbent and harder to dye. It is left as a “softer” thread than mercerized. And it’s absorbency can beneficial if sewing towels or other items that need to retain liquids.
Glazed
Glazed thread is coated with wax, starch and/or other chemicals. Glazing gives the thread a glossy smooth finish and higher resistance to damage, but can gum up your machine.
Gassed
Gassed thread has been subjected to high-heat to reduce fuzz. Yay, lint reduction!
Bonded
Bonded thread is treated/coated with a resin to increase strength and damage resistance. Typically this is applied to synthetic fiber threads including monofilament and corespun threads.
Anti-wick
Anti-wick treatment helps the thread repel water and resist mildew. It’s great for preventing “weeping seams” and is resistant to saltwater damage. This makes it good for water-based hobbies and sports.
Fire-Retardant
Fire-retardant thread is coated to resist extreme heat. It has a use in making fire-retardent clothing for everything from racing suits to children’s clothing including fire-breathers. And is beneficial for bedding and blankets for children too.
Understanding Thread Measurements
Let’s talk about thread sizes – and don’t worry if this seems confusing at first! I remember being completely baffled by all the numbers on thread labels when I first started researching.
There are three main ways thread is measured, but a good google search will turn up a few more systems floating out there.
The three most common measurement systems are:
- Weight
- Denier, and
- Tex
But, in some threads you might run into a Number System/Standard or Composition.
Weight, Denier and Tex all relate some quantity of thread to weight in grams or kilograms. The Number System and Composition are less specific in the measurement. Let’s dive in.
Weight
A thread measured in Weight (wt) will denote how many kilometers it takes of that thread to equal 1 kilogram. So, a heavier, thicker or denser thread will have a lower number because it takes fewer kilometers (less length) to get to 1 kilogram.
Thread weight will usually be denoted as a number and wt after, like 30wt. Which would mean it takes 30 kilometers to make a kilogram.
A typical all-purpose thread is 40wt.
Denier
The Denier measurement takes 9000 meters of thread and weighs it. So, in this case a heavier, thicker or denser thread will have a higher number because the same length of thread will weigh more.
Denier will be denoted: Denier 225 or whatever the denier number is.
A typical medium-weight thread is Denier 225.
Tex
Tex is much like Denier, except that it takes 1000 meters of thread and weighs that. Again, a higher number means a thicker/heavier thread.
Denier and Tex are easily converted by dividing Denier by 9 to get Tex, or multiplying Tex by 9 to get Denier.
You can convert Tex to weight by dividing 1000 by Tex.
And you can convert Weight to Tex by dividing 1000 by Weight.
Thread measured in Tex will be noted as Tex 30 or whatever the Tex number is.
I’ve seen all-purpose thread called out from Tex 20 to Tex 40, but typical medium weight will be somewhere between 27 and 35 usually. So you can think of Tex 20 or 24 as a finer thread and Tex 40 as a heavier thread than the typical medium weight.
Number System
So, the Number System is where things get… weird. The standard is a bit tricky and evidently was developed in Japan. A smaller number is a heavier thread, much like Weight. However, do not confuse the two they are not the same. If you see: No. 50 on the thread, it is not the same as 50wt.
But a No. 50 thread will be heavier than a No. 100.
Composition
Composition sizing will combine two measurements. You may see a 50/2 which would mean no 50 size and 2 plys of thread. 30/3 would be a number 30 size 3 ply thread. And often the size is given in the number system but some thread companies may use Tex or Denier instead of the number system.
Take all weight measurements of thread with a grain of salt. Just as it may be unknown to you (how these measurements work) or confusing, doesn’t mean all the thread manufacturers also know/knew what the measurements actually signified. So, if a measurement system was borrowed, it might not mean the same thing if the company that borrowed it didn’t do the research.
I think what is most important is if you are buying thread that is the same brand, you can generally count on it’s measurement system to be more consistent across it’s thread offerings. So knowing which measurement system they are using and how it works (does a larger number mean a larger thread? Or does it mean a smaller thread?). And If you see two brands using the same thread measurement system and you experience challenges with one of them and notice it’s a different measurement, it could be cause it really is a different size. This happened to me with regular serger thread from two different brands. I fought threads breaking for hours before I realized the “new” thread was larger so needed a lower tension.
Conclusion
Remember, while all this technical information is fascinating, don’t let it overwhelm you. For most projects, a good quality all-purpose polyester thread will work perfectly fine. The most important thing is to avoid the super cheap threads that can break easily and cause frustration. And keep the variety of threads available in the back of your mind for when (if) you actually need to use something other than all-purpose thread.