Thread Size

How big is a thread? It's pretty easy to measure the length, but how thick is it? What do those numbers like 30/2 and T-25 mean? As it happens, those numbers are all based on weight, not diameter. The sewing industry fundamentally doesn't measure thread diameters (monofilaments being an exception). So you have to know what a "fine" or "coarse" thread is based on those cryptic numbering systems. I will explain where they come from, what typical thread sizes are, and where to get more information.

There are some other critical questions for DE's. Are your suppliers and contractors using the same thread sizing system you are? Will everyone understand how to interpret "size 40 thread"? Because there are so many thread sizing systems in use, the potential for miscommunication is significant. I have some recommendations for avoiding misunderstandings when writing technical packages or style sheets.

Starting a couple of hundred years ago, every community of spinning mills had their own thread sizing system. Not just each country or region, but each fiber, as well. Today there are still quite a few surviving thread sizing systems. The cotton count system, the tex system, and the metric ticket system are the ones most often encountered in general sewing. I will mention a few others that still show up in niche applications.

All thread sizing systems are based on length and weight. There are fixed weight systems which measure how much thread weighs a given amount, and fixed length systems which measure how heavy is a given length of thread. These are sometimes called indirect and direct systems, respectively. A finer thread gets a larger number in a fixed weight system and a smaller number in a fixed length system, which is just one of the many sources of confusion about thread sizing.

There are a couple of reasons why threads are sized by weight. From the spinning mill's point of view, a pound of fiber is a pound of fiber. It has the same amount of raw material in it, no matter how coarsely or finely it's spun. Once mechanized ring spinning was introduced around 1830, the additional labor or wastage needed to spin more finely became negligible. And how, exactly, do you measure a thread's diameter? Making tiny measurements was much harder 200 years ago than it is today, and it's not trivial today. A thread is not a rigid object. Squeezing it in a micrometer will flatten it out. And (with the exception of modern monofilaments) it has a literally fuzzy boundary. If you look at one under a microscope, where does the thread end and the air begin? Will three different inspectors make the same decision? On the other hand, measuring the length and weight of a thread is very easy and accurate.

Thread weight for all of these systems should be measured for as-spun griege yarn. Dyes, sizing, lubricants and bonding agents can all increase the weight of the yarn, sometimes very significantly. It was not unheard of for unscrupulous mills to take the thread weight after loading the thread with various compounds. Depending on the thread sizing system, this would give the size corresponding to a finer or a stronger thread. Thread length should be measured when the thread is not under tension, as all threads have some amount of stretch. Loosely spun yarns can have an enormous amount of stretch.

Because thread sizing is based on weight, threads made from different materials can be different in size but have the same number (or have the same size but different numbers). That is true no matter which specific thread sizing system you use. For a given weight, there will be less fiber from dense materials than from lighter ones. This is actually noticeable when comparing extruded filament synthetics (nylon or rayon) against spun staple cotton.

Specific Thread Sizing Systems

The cotton count system is based on the number of 840 yard hanks you get from one pound of thread. You get 8,400 yards of size 10 (coarse) or 84,000 yards of size 100 (extremely fine) from one pound of cotton yard in this system. The size is measured for an individual yarn or strand. Most threads are made from multiple strands or plys. In the cotton count system, 50/2 designates a two-ply thread made from two size 50 yarns. That has the same fiber content as one size 25 yarn.

The cotton count system is sometimes called the yarn size system. Despite the use of arbitrary and archaic units, the cotton count system is still used heavily world-wide to describe fabric yarns as well as sewing thread. (Unfortunately, "Super 120s" and similar wool yarn sizing is the product of marketing deparments, not an actual sizing system. However, cotton count sizes are used for most other fabric yarns.) While this system grew out of the cotton spinning industry, its use is not restricted to cotton threads. Polyester and polyester-cotton threads are frequently sized in this system.

The Hong Kong ticket system is the same as the cotton count system, just written without the slash. So a cotton count 50/2 thread is a Hong Kong ticket 502 thread.

There are several derivations from the cotton count system that use a size without a ply count. Singles equivalent gives the single yarn size (1-ply) with the same weight as the thread being measured. For example, 30/3 is singles equivalent size 10, while 30/2 is singles equivalent size 15. Thread weight gives the yarn size of the 2-ply thread with the same weight as the thread being measured. For example, 30/3 is thread weight 20 (20/2 = 30/3), while 30/2 is thread weight 30. Thread weight is used commonly for embroidery threads. Cotton ticket gives the yarn size of the 3-ply thread with the same weight as the thread being measured. For example, 30/3 is cotton ticket 30, while 30/2 is cotton ticket 45 (45/3 = 30/2). You have to know which of these systems is being used, or your thread sizes can be off by a factor of three!

The Tex system is based on the weight in grams of a thread 1 kilometer long. A kilometer of tex 10 (very fine) thread weighs 10 grams, while a kilometer of tex 100 (very coarse) weighs 100 grams. The Tex system measures the entire thread, no matter how many strands or plies it has. While a thread can have any actual weight, the Tex system has official ranges of sizes that get the same Tex number. For example, all threads weighing between 24.0 and 26.9 grams per kilometer are designated T-24.

The Tex system is, in my opinion, the least ambiguous and most logical system to use when communicating a thread size in most situations. The ISO (International Organization for Standardization) is pushing Tex for national standard use world-wide. Suppliers and contractors who don't normally work in Tex should still be familiar with it. Of course, if you are sourcing hand-spun yak yarn, you are going to get what got made, no matter what you ask for. And there are some parts of the industry (embroidery floss, knitting yarns) that have not started using Tex measurements.

The metric count system is based on the number of 1 kilometer hanks you get from one kilogram of thread. If we were doing high-school word problems in math class, we would find that the metric count size is 1000 divided by the Tex size. So size 100 is a fine thread and size 10 is extremely coarse. This system is sometimes called thread weight, but should not be confused with the other thread weight system based on cotton count. Usually the metric count system is applied to an entire thread, but sometimes the individual ply size and ply count are given. So you can't assume 70/2 is a cotton count size. It could be a metric count size. Metric ticket gives the metric count yarn size for the 3-ply thread with the same weight as the thread being measured. In other words, metric ticket is three times the metric count for the whole thread. Metric ticket is very widely used.

The denier system is based on the weight in grams of a thread 9 kilometers long. Denier is most often used for continuous filament threads (especially synthetics) rather than threads spun from staple fibers. The name may be familiar from descriptions of nylon fabrics, 750 or 1000 denier "ballistic cloth" for luggage is woven from very heavy yarns, and 200 denier "packcloth" is woven from fine yarns. Denier measurements should obviously be nine times larger than Tex for the same yarn. However, denier is sometimes applied to individual strands of a multi-ply thread, while Tex is always applied to the entire thread. Denier sizes are usually written as 1000d, 200d, and so on.

Commercial sizes for heavy-duty thread used in upholstery and webbing (e.g., safety rigging and vehicle tow straps) are 33, 46, 69, 92, 138, 207, 277, 346, 415 and 554. These sizes are the denier measurement divided by 10. (A size 554 thread is about 1/32" or 0.8mm in diameter, which is as large as some decorative trims!)

Linen count and wool count sizes are related to the cotton count system, but they use different units of length. In fact, the wool count system never really agreed on how long one skein or hank was. 256 yards was common, but both larger and smaller units were used. Today, different units are used for woolen and worsted yarns! The linen count, which still may be encountered in specialty niches like heirloom sewing and lacemaking, is based on 300 yards. Like cotton count, these systems are applied to individual strands or yarns, and the number of plies is shown separately.

Silk machine twist sizes are another fixed weight system, but the sizes are given as letters rather than numbers, ranging from 000 through A through F and then to FFF. Exactly what length a given letter means was always somewhat arbitrary, based on the manufacturer of the thread, but there is reasonably good agreement on the common sizes A, B and F. Very roughly, size A is 900 yards per pound of thread, and every 100 yards difference is one letter size different. This system is applied to the entire thread, not individual plies. Today this system is found mostly in hand tailoring (where it applies to nylon and silamide threads as often as actual silk). Size A is a general-purpose thread, size B is for heavy seams, and size F is for worked buttonholes.

There is another letter-based thread size system used in US government contracts. Its sizes are very roughly the same as the silk machine twist system, which makes it easy to confuse the two. In this system, size 00 is about T-16, size A is about T-24, size AA is about T-30, size B is about T-45, size E is about T-70, and size F is about T-90. These sizes are controlled by formal specification, not the whims of thread makers; unfortunately, I don't have a reference to the actual spec.

Tire, YLI and other contemporary silk thread makers use a numeric, fixed weight system that I have not pinned down. A size 100 silk thread is very fine, similar to T-10, while a size 15 silk thread is roughly T-40. If anyone knows the length unit for this 1 kilogram fixed weight silk system, I'd appreciate the information.

That by no means exhausts all the thread sizing systems still in use, but it's exhausted me. So let's boil all that down into a table comparing the systems just described. It's worth repeating that the cotton count, Tex, metric ticket, and perhaps denier, systems are the principal ones you will see, unless you are in a specialty area.

Thread Sizing Systems
  Symbols Type Constant Unit
Cotton Count
Ne, NeC, NeB
fixed weight
1 pound
840 yards
Tex
TEX, T
fixed length
1,000 meter
1 gram
Metric Count
Nm
fixed weight
1,000 gram
1,000 meter
Metric Ticket
Tkt
fixed weight
1,000 gram
333 meter
Denier
Td, d
fixed length
9,000 meter
1 gram
Linen Count
NeL
fixed weight
1 pound
300 yards
Wool Count
NeW
fixed weight
1 pound
256 yards
Silk Machine Twist
fixed weight, A=900 yards, B=800 yards, F=300 yards, etc.
1 ounce
100 yards

The number of thread sizing systems makes a miscommunication between you and your suppliers and sewing contractors very possible. There are several things you can do to ensure mutual understanding.

How Various Sizes Relate

Rather than give formulas to convert between all these systems, I've put together a small table with five typical thread sizes. Each column describes one real-world thread, showing what the various thread sizing systems would call that same thread. The needle sizes are American & Efird's recommendations. For something less than full-speed production, you might be able to use one size smaller, but I have seen recommendations even larger than A&E's so don't get carried away.

Sample Sizes
  Very Fine
(Blindstitching)
Fine
(Heirloom Sewing)
Medium
(General Sewing)
Coarse
(Topstitching)
Very Coarse
(Hand Buttonhole)
Cotton Count
80/2, 120/3
60/2, 90/3
30/2, 45/3
17/2, 25/3
12/2, 18/3

Tex

T-15
T-20
T-40
T-70
T-100
Metric Ticket
200
150
75
45
30
Denier
135d,
68d x2, 45d x23
180d,
90d x2, 60d x3
350d,
175d x2, 117d x3
630d
315d x2, 210d x3
900d,
450d x2, 300d x3
Linen Count
225/2, 335/3
165/2, 250/3
85/2, 125/3
45/2, 70/3
35/2, 50/3
Silk Machine Twist
-
000
0, A
E, EE
F
Appropriate Needle
9 / 65
10 / 70
14 / 90
18 / 110
20 / 125

I would like to include another item from American & Efird here. Thread size recommendations are usually given in terms of application (e.g., lingerie, dress shirts, pants, work clothes). A&E's list is based on fabric weight, instead. (Errors in the metric weights would be my fault.)

Thread Size Recommendations
Fabric Weight (oz/yd^2) Fabric Weight (g/m^2) Thread Size
2 to 4
65 to 135
T-16 to T-24
4 to 6
135 to 200
T-24 to T-30
6 to 8
200 to 270
T-30 to T-50
8 to 10
270 to 340
T-40 to T-60
10 to 14
340 to 475
T-60 to T-135

References

There are lots of good references on the web if you want more detail. As you might expect, some of the best are provided by thread manufacturers.

Great Western Sewing Machine Service has a short summary of how thread and needle sizes are related.

UK Sewing Services aka Supply Division has a cheatsheet page with thread consumption estimates.

Here are some pages with conversion formulas between various thread sizing systems.