The Fixed Weight Yarn Count System is used for other fibers besides wool. Counts for cotton, linen and spun silk yarns are also measured on the Fixed Weight System (FWS).

Cotton:

The standard is 840 yards of singles yarn per 1 lb of cotton lint.
The number given to the spun yarn denotes the length of the yarn so:
#1 cotton is 840 yards/lb
#2 cotton is 1680 yards/lb
#3 cotton is 2520 yards/lb
#10 cotton is 8,400 yards/lb.
#20 cotton is 16,800 yards/lb.

If the cotton yarn is plied, it is noted like this:
10/2 cotton. This means that 2 threads of #10 cotton singles spun at 8,400 yards/lb have been twisted together to give 4,200 yards/lb of 2 ply yarn.
The first number is the size of the singles yarn, the 2 gives the number of plies.
So... 10/4 cotton would be 4 threads of #10 cotton twisted together to give 2,100 yards/lb.

Do not confuse the FWS for the count of cotton yarns with cotton yarns numbered using the NM System. NM stands for number of meters and is based on the number of meters in one gram of yarn. The NM designation would be noted either before or after a number indication the number of meters in 1 gram of yarn: NM 10 or 10 NM meaning there are 10 meters of yarn per gram. Plied yarn would be marked NM 2/10 with the first number indicating the number of plies and the second number giving the meters per gram.

Linen:

Flax fibers undergo a name change when they are spun and become linen. The unit of measurement for linen yarns is the lea and 300 yards per 1lb of fiber = 1 lea.


Silk:

In the English system, spun silk count is based on 840 yards per 1 lb of fiber.
#1 silk is 840 yards/lb
#2 silk is 1680 yards/lb
#3 silk is 2520 yards/lb
#10 silk is 8,400 yards/lb.
#20 silk is 16,800 yards/lb.
While the silk yarns may have multiple plies and skeins may be marked with the number of plies, the size number will always remain the same. So a #10 silk yarn will always have 8,400 yarns no matter how many plies went into making that yarn.

Calculating Cotton, Linen, and Silk Yarn Counts:

The formula for calculating the count of cotton, linen, and silk yarns is the same as those for woolens and worsteds:

Number of yards divided by weight times 16 oz (454 grams) times the number of plies divided by the FWS factor for the specific fiber equals Count.

The FWS factors are:
1. Cotton – 840
2. Linen – 300
3. Silk - 840