Thread numbering is a complex subject. Often, each manufacturer has their own habits and there can be differences between one company and another.
Even if the basic calculations are the same, they have evolved over time and not all the manufacturers have followed this evolution. It proves even more difficult in Great Britain where both metric and imperial measures are used.
Add to that the calculations to find the yarn count which differ depending on the type of thread.
In principle, France uses metric numbering (NM) since 1942 for cotton yarn: a thread measuring 1,000 metres
with a constant weight of 1,000 grams. Thus a N°1 is a thread which weighs 1000 grams for 1,000 metres and,
for example, a N°40 is a thread weighing 1,000 grams for 40,000 metres.
Before 1942, French numbering was used, where 1,000 metres of thread would weigh 500 grams.
For English cotton yarn count (NE), the calculation is based on 840 yards (768.1 metres) for 1 pound (453.6 grams). To convert the English number into metric, it must be divided by 0.5905…
Concerning combed wool, metric numbering is principally used, but old habits die hard.
In the Alsace region of France, they calculated on a base of 700 metres for 500 grams whereas in the north of France it was 720 metres for a weight of 500 grams…
Yarn count for linen thread is based on the English (NE), so a N°1 is 300 yards (274 metres) weighing 1 imperial pound (453 grams). But that would be too simple…
Traditionally, linen thread was sold by packets of 100 hanks, a hank being made up of 12 skeins. So as an example, a packet of 100 hanks N°1 contained 360,000 yards of thread (329,184 metres) and weighed 543.6 kilos.
However, in France a conventional weight of 540 kilos was adopted!
So calculation time again… A N°36 in English count is 36 x 10/6 = 60. And the other way round, a N°40 in French numbering is 40 x 6/10 = 24.
And again for silk it is different: the length unit is 450 metres and the count is expressed in demi-decigrams
(5 centigrams), also called denier.
There is a different method for calculating the number of grams for 9,000 metres of thread. If a silk thread measuring 450 metres weighs 1,500 grams, the count (TD) is calculated 1,500/0.0550 = 30 deniers.
If the hank of 9,000 metres weighs 150 grams, the TD is 150 deniers.
Now, to convert the NM into TD, use this calculation: NM ÷ 9,000/TD or 9,000/NM for the inverse.
An example: A NM 60 is 9,000 ÷ 60, so 150 deniers.
For nylon threads, it is the same calculation as for silk, whereas for rayon, the NM is used.
So in brief, you will understand that thread numbering, even though an exact science, varies in methods between producers, as much for historical reasons as traditional and regional…