Sewing thread sizes and how to tell the thread size

In this sewing tutorial, I will talk about sewing thread sizes and how to choose a correct thread size for your project.

A printable version of this article is available on my Etsy shop.

The subject of sewing thread sizes is a vast one but the one you need to understand if you want to master sewing.

Because of the diversity of materials and construction methods, it’s not easy to categorize the threads. And a thread is always used (well, almost always) in combination with a needle, so at some point, we have to deal with the thread size together with the recommended needle size.

Sewing thread sizes

Why is it important to know the sewing thread sizes and when we really need to pay attention to the thread size? I think you have read many times this recommendation: “for lightweight fabric use a thinner thread, for heavyweight fabric use a thicker thread”. But the question is – how do we determine what thread is thinner and what thread is thicker? 

Also, if you want to sew nice seams without seam puckering you should use the same size thread in the bobbin or thinner because if you use thicker thread in the bobbin you can get puckers in your seams.  

We all know that the thread tension on a sewing machine has to be set depending on the thread size. When we use a thicker thread for sewing we need to decrease the tension a bit, and when we use a thinner thread for the lightweight fabric we need to increase the tension for the stitches to look nice. 

If you are using the wrong size thread you may have a jam in the machine, the thread may break or make a “caterpillar”. 

So I decided to look at the threads I have at home and see how they are numbered. And you know what? I was completely confused by all that information. 

In the image below you can see a multitude of symbols that define the thread size. And all the threads in that image appear to the naked eye to be about the same size, nevertheless, they could not be more different!

Let’s see what we have, and I will explain later what each notation means. I have all these different spools at home and couldn’t be more confused until I decided to study the problem and learn all the information about thread sizes. 

This thread is labeled 120D/2. This means 2 strands twisted together, each strand 120 denier for a total of 240 denier for the thread. If there was only one strand, the notation would have been 240D/1. Sometimes the “1” is omitted so you may encounter simply “240D”.

The label 50S/3 denotes a thread of weight 50 made out of 3 individual strands of material (polyester in this case) spun together

This label 27 wt denotes a thread having the weight 27.

Similar to the one above, this is a thread of weight 60. And with the naked eye, I don’t see a huge difference in thread diameter from the one above! Weight does definitely NOT always mean thread thickness!

This shows the thread is a polyester thread of weight 60 made out of 3 individual strands twisted together.

Den 75/2”. Is this a secret password for a new game? No, this is a denomination for an embroidery thread size, it means 2 strands twisted together, each strand 75 denier for a total of 150 denier. We have met this type of notation before, but it was 120D/2. Both mean the same thing, said differently.

This thread says “No 50”. This can also be expressed as “metric 50”. 

No. 120. This thread SHOULD be the same thing as “metric 120”. However I personally have real doubts this is what the number means and I presented it to show that even when the manufacturer does put a label, you need to be careful! It may mean other things than you thought.

The label “tex 30” introduces another measurement type “tex”. Not better or worse than what we have seen, just different.

Tex. can be expressed also as T only followed by a number; this label means the same as “tex 52”.

There are more units, I just selected a couple as examples. Let’s see what each means and I will then get to actually prove that the threads, which at a first glance loom the same size, are 100% different. Appearances can be deceptive!

Units used for thread size classification

The most common units used for classifying thread sizes are Weight (wt), Denier (den) and Tex. These are not the only units though. There are a number of comprehensive articles on Wikipedia (Thread (yarn), Units of Textile Measurement) which you may want to consult for more obscure units. 

Weight

Weight of a thread is the length (in kilometers) of a length of thread that weighs 1 kilogram. It shows how many kilometers of thread are needed to make 1kg. If weight is 40, it takes a length of 40km to make 1kg of thread. Lower the number, heavier the thread.

thread weight 60

Weight for thick, heavy threads is a lower number (8, 10, etc). For thin, light threads it’s a higher number (50, 60). 

Ironically, the thread weight number which is derived from km/kg is used in North America while all US dimensions are still in inches/feet/yards! 

The paradox here: heavier weight has a lower number! It makes sense though, the heavier the thread, the shorter the length necessary to make 1kg. The lower the number, the thicker and stronger the thread. Usually. But not always. Let me explain.

The sewing thread diameter depends on thread density and material (it’s made of) too. I have seen instances where a thread marked with weight 30 is THINNER than a thread marked with weight 40. You can find threads with weight 50 that are STRONGER than threads marked weight 40. 

Density gives the thickness of the thread, and that depends both on the material and the construction method. A lower weight number (i.e. greater weight) doesn’t always mean a thicker thread or a stronger thread.

In the images above I have 2 very different threads, one is all purpose thread polyester weight 40, and the other is an Eloflex thread (a stretchable thread that can be used on a regular sewing machine for sewing stretchy knits) weight 27. Both these threads look the same in diameter/thickness to my naked eye. Why? Because Eloflex thread is made from HEAVIER fibers (I guess, they use elastane) so it takes only 27km to make 1 kg of the thread. 

Sometimes, not always, the thread weight (WT. abbreviation) is followed by a forward slash and a number. That number signifies the number of individual strands of thread that are twisted together to make the final product. 

The weight followed by the number of strands is in fact the “Gunze count” of the thread, which is an obscure Japanese measurement system from which the weight derives. The following table is imported directly from the Wikipedia article I cited above. 

Denier

This measurement is the weight in grams of 9000 meters of thread (9km). If 9,000 meters weighs 70 grams, it’s a 70-denier thread. Larger denier numbers are heavier threads.

A little bit of history: 

Not so long ago, before fine lycra fibers were available, silk stockings were darned as a rule. There were in fact special shops and special tools for doing this (this darning mushroom image is from Wikipedia “Darning” article). It was very important then to get the exact thread size for darning because otherwise, the repair would have been very visible. It was in fact customary for silk stockings to have some thread in the package exactly for this purpose, so the “denier” measurement was used and is a valid unit for sewing threads. 

Is Denier thread size really still used at all for sewing? Yes, it’s still used to this day, as you can see, even if not for regular sewing thread, but mostly for embroidery thread.

Tex

The “Tex” number is the weight in grams of 1000m (1km) of thread. I couldn’t find a reference to the origin of the name “tex”, as opposed to “denier” (which seems to originate in the name of an old French coin). 

However, tex has been adopted by the International Organization for Standardization (IOS) as the measurement unit for thread density and as such is probably going to increase in popularity. But given that foot, inch, mile and so on have been replaced by the same organization since 1875 with the meter and we still use them 150 years later, I wouldn’t put too much trust in the fact that “Tex” will become generally accepted any time soon. 

However it’s in use, especially in Canada and Europe, and it’s not unusual to see threads marked in “tex” units even in the USA. Below are some examples from my collection.

The finer the thread (thinner, lighter), the lower the tex number. We could categorize them as:

  • Fine Tex Threads . . . . . . Tex 9 to Tex 19 (thin, embroidery, special)
  • Medium Tex Threads . . . Tex 20 to Tex 69 (regular)
  • Heavy Tex Threads . . . . . Tex 70 and higher (heavy duty, upholstery)

Now you know what all these numbers on threads mean and hopefully, you are not going to be so confused anymore by all the different numbering systems for the sewing thread size. 

So, what would a “regular size” (for example) thread mean? For a thread marked in weight, it would be a weight 40. For something labeled in tex, it would be somewhere around tex 25, and for threads marked in denier, it would be 225. With the note that only embroidery thread is labeled in denier.

A thinner thread is usually weight 50-60 and higher, tex 20 and lower, denier 180 and lower. But remember about the exceptions I mentioned above. 

And a thicker thread is usually weight 30 and lower, tex 30 and higher, denier 300 and higher

But remember that threads marked differently in Wt. (or Tex.or Den.)numbers can have the same thickness (see the example above with the Eloflex thread). The thread thickness depends on the density of the thread, and the fibers used.

I think it’s good to know how they categorize threads by their sizes. We can be confused by all these different numbers but in reality, it’s not so difficult to choose a correct thread size for your project. 

Often they don’t even put thread sizes on spools. I have plenty of brand name threads that don’t have markings for the thread size. For example, Gutermann threads are not marked with size.

In this case, we choose the thread by application and fiber

If it’s an all purpose thread – it’s most likely wt. 40. 

If it’s a silk thread it’s probably good for lightweight fabric (and I can say I used it numerous times for my silk projects) – is it thinner than polyester thread? Who knows, the spools I have don’t have the size marks. 

If it’s written heavy duty it should mean that the thread is thicker. 

Serger threads are generally thinner than the regular sewing thread because serger stitches take a lot of thread to form the stitch so if you have cones marked as serger threads you should know that they are thinner threads. 

So if you have a collection of threads that don’t indicate what sizes they are, just use your eyes: take the thread in your hand, maybe try to break it, place it on a piece of fabric and see how it looks, compare different threads and compare the one without markings for the size to others that have the size marked. Often you can see how thick or thin the thread is although the thickness of the thread can be deceiving when you look at it. 

Just for your information, there are many other measurement units for the sewing thread sizes ( in addition to weight, tex, and den.)  but I only want to mention one more because I did encounter it and I had no idea where to take it from: ticket number. This is in reality not something the home user should be concerned about, the “ticket number” for a thread is only an internal reference number the manufacturer uses to identify the thread. 

There are complicated formulas that can derive the denier, tex, or weight from this, an excellent explanation is in this article. The “ticket number” system is not unique, there are many systems of numbering the threads like this. Suffice to say, this is used mostly for industrial threads, good to know it exists, glad I can ignore it!

Because we have reached this point, there are more units that can safely be ignored and I will mention them here just for reference; they are used almost exclusively in industrial sewing. Just a few: Metric ticket number (Nm), Cotton Count ( NeC, NeB, or Ne), Hong Kong Ticket (sometimes named “far East”), decitex, and others.

While we are talking about sewing thread sizes I need to mention the quite obvious fact that threads are also different in length – we have small spools and big cones. What to choose?

Threads come generally in a limited number of packages, mostly depending on the amount of thread. I am talking here about ranges in “meters” and “yards”. Threads are not always marked with their size in Wt., Tex. or Den. But they always say how many meters/yards are there.

You can usually see a clear difference in thread thickness if you compare two spools of thread that are the same size. For example, in the image below you can see the same size spools but the spool to the left (with brown thread) has 110 yards of thread, and the spool to the right has 274 yards, and this tells us that the green thread is much thinner. 

Thread packages

Spool: holds a small amount of thread, around 250 yards or less usually. Not used for industrial machines, the small diameter makes it unsuitable for high speed sewing but widely used in home sewing machines. A spool is generally under 3” (7.5cm) long, the one here is about 2”x3/4” (5cmx2cm)

A cop is not only a policeman but also a thread wound on a solid core (plastic or paper) into a shape like a cylinder. Used normally for amounts of threads between 500 – 3000 yards, it’s easy to use, it goes easily on the sewing machine and unwounds easily. More economical (price per length) than a small spool.

A vicone is wound on a support with a wider base to allow it to sit better on support. The tapered top prevents accidental unwinding which is a problem with a “cop”. Used generally for lengths of thread less than 5000 yards. 

A cone is used for amounts of thread over 3000 yards and large cones can even go up to 25000 yards. A cone will probably not fit on an internal sewing machine support but will fit on the external stand some sewing machines have. Sergers use almost exclusively cones because of the large amount of thread used. A cone can support high speed sewing and is equally used for industrial sewing machines. 

There are other package shapes (container, cocoon, large package) but all are exclusively used in industrial sewing so of little interest for us here.

 

Industrial sewing machine thread sizes

 

Threads used for industrial sewing machines (industrial sewing thread) are generally categorized differently than for home sewing and below is an example from the Toledo Industrial Sewing Machines Ltd. Notice the only familiar notation is “Tex”. The chart compares various bonded nylon (or polyester) thread sizes and combines all in an easy to read sewing thread size chart:

The V size is the common US measurement for twisted, multi-ply bonded nylon or polyester threads. Larger numbers indicate heavier threads.

The T sizes represent the “Tex” measurement system, where the number equals the weight in grams of 1000 meters of thread. If 1,000 meters weighs 70 grams, it’s a Tex 70 thread. Larger numbers indicate heavier threads.

Please note that the “Tkt” (Ticket) sizes are equivalent to Metric “M” thread sizes used in some countries. Smaller numbers indicate heavier threads.

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Sewing thread sizes how to choose

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