More of my class notes, assembled from various sources.
Yarn thickness is determined by the number of times the standard length of yarn is spun.
The thickness number is calculated using the thickness of a single strand of spun yarn.
The number of spinnings and the number of plies (strands) produce the yarn ‘count’.
The thinner the yarn, the higher the number and the more yards to the pound.
These guidelines were provided for knitting using Jaggerspun yarns. I have no affiliation with the company. I do still continue to use some of the lines and have a great appreciation for all their yarns. Their wool used to be the required fiber in my knit labs at RISD until students began to understand some of the basics in stitch construction. I have permission from the company to share content. Superfine merino and zephyr wool silk are both 2/18.
A few more bits:
fixed length of yarn spun from a specified weight of the material
1’s 1 spinning
1’s Yorkshire woolen 256 yards per pound
1’s linen 300 yards per pound
1’s worsted 500-560 yards per pound primary standard for wool and acrylic
1’s cotton 840 yards per pound
1’s spun silk840 yards per pound
2’s mean that twice the above length of yarn was spun from one pound of raw material
yardage reference: the first number refers to the number of plies twisted together to form the yarn, the larger the second number, the thinner the spun strand; conversely the smaller the second number, the thicker the diameter of the strand
yardage may vary with plies depending on mill and country where the yarn is spun
2/24 5500 to 6700 yards per pound
3/15 2650 yards per pound (530)
2/15 3670 yards per pound (489)
in cotton, the markings are reversed
3/2 Cotton 3 x 840 divided by 2 = 1,260 yards/pound
20/2 Cotton 20 x 840 divided by 2= 8,400 yards/pound
one standard for hand knitting yarn: gauge is per inch, not for a 10 X 10 cm or 4 X 4-inch swatch
fingering = 5 or 6, knits approximately at 8 sts/12 rows per inch
sport weight yarn = 3 or 4, knits approximately at 6 sts/8 rows per inch
worsted yarn = 2, knits approximately at 4 sts/ 6 rows per inch
3/15 machine knitting yarn: the second number by the first = 5, so this would knit at approximately the same gauge as fingering weight
2 strands 2/24 = 4/24 = 6 = fingering weight
3 strands 2/24 = 6/24 = 4 = sport weight
4 strands 2/24 = 8/24 = 3 = heavy sport weight
5 strands 2/24 =10/24 = 2.4 = close to worsted
6 strands 2/24 =12/24 = 2 = worsted
for more information and convenient conversions for use in the machine, knitting see http://www.cara4webshopping.com/freebies_for_fun/yarn_weights.htm
the American Craft Council standard yarn weight system is included in their PDF on standards and guidelines for crochet and knitting http://media.craftyarncouncil.com/files/CYC_YS_s_and_g_rev2015_6.pdf