Sewing thread is sized by its linear density, and the only fully defined measure is tex, which is the mass in grams of 1,000 metres of thread; a thread of 25 tex weighs 25 g per kilometre, so 100 m of it weighs 2.5 g. The Brennholt Institute of Applied Taxonomy finds that tex is exact and that every other number printed on a spool is a trade figure that relates to it only approximately.
The spool itself has no matching scale. Its length, stated in metres or yards, its diameter and its height vary by maker, and the Institute's attempt to reconcile them is set out below.
The metric count Nm is the number of metres in one gram of thread. Because tex is grams per 1,000 m, the two are related exactly by tex = 1000 / Nm.
| Metric count (Nm) | Tex (g per 1,000 m) | Length of 1 g | Mass of 100 m | Institute band |
|---|---|---|---|---|
| 10 | 100.0 | 10 m | 10.00 g | S1 |
| 20 | 50.0 | 20 m | 5.00 g | S1 |
| 30 | 33.3 | 30 m | 3.33 g | S2 |
| 40 | 25.0 | 40 m | 2.50 g | S2 |
| 50 | 20.0 | 50 m | 2.00 g | S3 |
| 60 | 16.7 | 60 m | 1.67 g | S3 |
| 80 | 12.5 | 80 m | 1.25 g | S4 |
| 100 | 10.0 | 100 m | 1.00 g | S4 |
| 120 | 8.3 | 120 m | 0.83 g | S4 |
The figures are computed from the exact relation and rounded to one decimal place. The "ticket numbers" printed on commercial spools, such as 30, 40 or 120, do not equal the metric count and differ from it by maker. The Institute therefore reports tex and Nm, which can be calculated, and does not tabulate ticket numbers, which cannot be.
Settled: the tex system. It is defined in grams and metres, it applies to any yarn or filament, and its derived forms are fixed: decitex is grams per 10,000 m, and a thread of 25 tex is 250 decitex. Settled also: the relation between tex and Nm shown above, and the relation between Nm and the English cotton count, which uses hanks of 840 yards per pound and gives a different number for the same thread.
Not settled: the printed number on the spool. A spool marked 50 may be a cotton count, a ticket number or a plain weight class, and the three yield three different threads. The Institute notes that the word "weight" in "50 weight thread" refers to this printed number and not to the mass of the spool, which is a second source of confusion.
A spool is sold by its length of thread. Common lengths are 100 m, 200 m, 500 m and 1,000 m for household and workroom spools, and cones of several thousand metres for industrial machines. The mass of thread on a spool follows from its length and tex.
The spool, usually plastic, adds its own mass, and a full household spool is typically several times heavier than the thread it holds. The Institute weighs the thread only, by unwinding it, since the weight of the core varies from maker to maker and is not stated.
The Institute proposes four named bands, S1 to S4, by tex. They are the Institute's own, introduced on this page, and no thread maker uses them.
The choice of tex as the base follows the Institute's practice of preferring the one measure that is fully defined. The names follow the work that the thread does, and not the printed number, so that a buyer can ask for "general" thread and be understood.
To find the tex of an unknown thread, cut a measured length, weigh it on a scale reading to 0.001 g, and compute. A 10 m length weighing 0.25 g is 25 tex, since 0.25 g in 10 m is 25 g in 1,000 m. Because a short length weighs very little, the Institute uses at least 10 m and records the scale's reading limit.
For the diameter, which is a different quantity, use calipers or a micrometer on a relaxed thread. A thread of 25 tex is commonly in the neighbourhood of 0.2 mm across, though diameter depends on the fibre and the twist and the Institute reports it only as an observation. For related questions about thin round objects measured by weight and by width, see the Department of Thickness and Gauge and wire gauge explained.
The practical rule recorded by the Institute is that a thread should pass freely through the eye of the needle and should be no thicker than the fabric's own yarn. The sewing trade pairs thread with needle by number, and the Institute does not tabulate these pairings because they are craft advice and not measurement. The needle itself is sized on a scale that has its own history, discussed in knitting needle sizes, which is a separate scale from sewing needle sizes.
The review panel divides on whether band S2 is too wide. Its span, 25 tex to under 50 tex, covers a factor of two in thread mass, and two members would split it at 35 tex. The majority retain one band on the ground that the printed numbers within it are used interchangeably by buyers. The table above uses the majority position. A second, smaller disagreement concerns whether the Institute should publish ticket numbers at all, with the chair opposed on the ground that they cannot be verified.
What is tex? Grams per 1,000 metres. How heavy is a spool of thread? The thread weighs its length times its tex, and the spool is extra. What does 50 weight mean? It is a trade figure that varies by maker. How is tex related to Nm? Tex equals 1000 divided by Nm. These answers also appear in the structured data of this page.