In the UK until the mid twentieth century knitting yarns were 2 ply, 3 ply, 4 ply or Double Knitting (four thicker plies, equivalent to 2 x 4 ply) and occasionally very thick yarns such as Aran or chunky. You used size 12 needles for 2 ply, size 11 for 3 ply, size 10 for 4 ply and size 8 for DK and two sizes smaller for the ribbing. Needle sizes were based on Standard Wire Gauge (an indirect system of measurement with high numbers for thinner needles) and everyone knew what was what. Although the various manufacturers didn't like to acknowledge it one brand of 4 ply could be substituted for another; you just knitted the tension square to be sure.
In Australia and New Zealand similar yarn sizes were used but for the thicker yarns they had 5 ply, 8 ply and 12 ply. Not usually that many actual plies but a nominal equivalent relating to regular 3 ply and 4 ply.
In America semi-descriptive names like Baby, Sport, Fingering and Worsted(weight) were, and still are, used (worsted really means a woollen yarn which has been carded and combed, rather than just carded, to make the fibres lie parallel to each other and has nothing to do with size). American knitting needles sizes are measured by a direct system of measurement with high numbers for thicker needles, though the standard varies a bit from manufacturer to manufacturer. No wonder there is confusion!
Needle sizes are getting standardised now with both sides of the Atlantic using metric sizes, which are measured by the diameter of the needle, so the higher the number the thicker the needle.
Since WW2 a large proportion of the wool yarns have been substituted with acrylic and other synthetic fibres, and often even very thick yarns have only two plies; fewer plies = less spinning, therefore less expensive to manufacture. Also there are now numerous fancy yarns available such as boucle, slub, knop, chenille, eyelash, pom-poms etc. These are mostly fashion yarns, any individual brand might only be around for one or two seasons. All of this means that it can be very difficult to assess the thickness of a yarn by description alone.
The Craft Yarn Council of America has tried to standardise hand knitting yarns into seven weights, but inevitably some yarns fall between the two, and the various charts produced by knitters sometimes place the same type of yarn into different categories. The following table shows the categories into which different types of yarn might be placed. The table is also rather deficient in that there is no size 00 (Fine-lace or Cobweb) to account for the very fine Shetland singles wool and similarly fine synthetic yarns which are usually sold for machine knitting.
Machine knitting and weaving yarns are often labelled with a size.
Metric number, Nm, is the most frequently used, but sometimes Tex or dTex is used and sometimes a wool count or cotton count equivalent is quoted.
Nm = number of 1,000 metre skeins obtained from 1 Kg fibre
Tex = weight in gms of 1,000 metres of yarn; dTex = weight in gms of 10,000 metres of yarn
Nm is an indirect size, the higher the number the finer the yarn. Tex and dTex are direct sizes, the higher the number the thicker the yarn.
Although two yarns with the same Nm contain the same weight of fibres per metre, spinning variations can affect the finished product. A tightly spun yarn will appear finer than a loosely spun yarn. Also different fibres have different densities which means that yarns with different fibre content cannot always be directly compared by the size number. Cotton and linen fibres both have a specific gravity of 1.54, silk and wool fibres have a specific gravity of about 1.32 whilst acrylics and other synthetics have specific gravities of about 1.14 - 1.18. Thus 1 Kg of woollen fibres will yield a longer length of yarn of a given Nm than 1 Kg of cotton or linen will, but a shorter length of yarn than 1 Kg of acrylic fibres will. Compare two similar garments; one made of cotton and the other of a sythetic. The cotton one will weigh more than the synthetic one.
The reality is that there is no truly meaningful size system for hand knitting yarns. Comparing yarn thicknesses by wraps per inch (wpi) is the most practical, especially for lacemakers who want to use thicker or novelty threads for lacemaking, and knitting up a tenson swatch is the only way to know if a particular yarn will knit up to the same size as quoted for a specific pattern.
The knitting & crochet forum Ravelry have a lot of general information about knitting yarns, and on their wiki pages the following info is included
|UK, NZ, AU name
|Knit guage - 4inch/10cm
|0 : Lace
|0 : Lace
|0 : Lace
|0 : Lace
|1 : Super Fine
|2 : Fine
|3 : Light
|4 : Medium
|4 : Medium
|5 : Bulky
|6 : Super Bulky
My experience of making windings with knitting yarns always comes out with a higher wpi than those quoted by Ravelry; ie four-ply /sock yarn is quoted as 14 wpi but my windings for 4 ply sock yarns are consistently 18-19 wpi . Also tthe Ravelry description Thread is pretty meaningless as any lacemaker will know!
I have measured a number of smooth spun knitting yarns from my stash. Mostly UK brands and many without the original labels, but the results were quite consistent.
|Ravelry Descriptive Name
|Wraps to my winding tension
|1 ply / Cobweb
|Shetland wool singles
|14-15 w/cm = 36-38 wpi
|2 ply / Laceweight
|2 ply Shetland laceweight
|9-10 w/cm = 23-25 wpi
|3 ply (baby) / light fingering
|3 ply baby (UK brands)
|8 w/cm = 20 wpi
|4 ply (sock) / fingering
|4 ply equivalents (UK brands)
|7 w/cm = 18-19 wpi
|Double Knit / DK
|DK (UK brands)
|6 w/cm = 15 wpi
|Aran (UK brands)
|5 w/cm = 12-13 wpi
I have also measured a few acrylic machine knitting and other yarns where an Nm size is quoted.
|Shetland 2 ply lace weight wool
|Faulkland 4 ply equivalent wool
|Acrylic 4 ply equivalent
|Shetland 4 ply equivalent wool
In Threads for Lace I used wraps per cm to compare thread thicknesses because most lacemakers choose to use finer threads, but for those who wish to use thicker threads, such as knitting yarns it is more accurate to measure in wraps per inch. The conversion is simple:
Wraps/cm = wraps/inch ÷2.5 wraps/inch = wraps/cm x 2.5
This is very convenient if you are looking to use a knitting yarn as the gimp for a piece of bobbin lace because the ratio in wraps.cm of
main thread: gimp thread = 2.5 : 1
Thus you choose a main thread with the same number of wraps/cm as the wraps/inch size of the knitting yarn.