Yarn Count Explained:
Understanding the labels on yarn packages
This is an excerpt from Peggy’s new book, the revised edition of Book #1: Winding a Warp and Using a Paddle, that just came out in July. This is information not in the first edition and comes in the extensive new chapter on Sett. 
Table of Base Counts of Threads

Yarn CountThere are official standards for measuring the number of yards in one pound of yarn. These standards were created by cloth manufacturers in the nineteenth century. The standards refer to the weight given to a given length of thread. For cotton, they used a reel with a circumference of 1 1/2 yards or 54″; for linen, one of 2 1/2 yards or 90″; for woolens, one of 2 yards or 72″. Silk uses the cotton reel. A 1 ply thread of cotton wound around its reel = 1 1/2 yards; 80 threads = one skein or 120 yards; and 7 skeins = one hank or 840 yards or one pound! So, now we know that there are 840 yards (one hank) of one ply cotton thread in one pound. This is the base measure or the “onecount.” Linen is calculated in leas, where 1 lea = 300 yards of one ply linen = 1 pound. Worsted is measured by counts. One count = 560 yards of single ply worsted = one pound. See the Table of Base Counts of Threads for some other base measures (onecounts). Now, it is obvious that a very thin yarn will take more yards to weigh a pound than a thick yarn would. On the package or cone of yarn is a fraction which tells you how your yarn relates to the base measure. It also tells you how many plies the yarn has. You need to know this because the measure of yards per pound is based on one ply thread. Say your package of cotton thread has a fraction of 3/2, for example. The top number tells you the size of your yarn and that it is three times the base measure or 3 x 840 = 2520 yards per pound. The bottom number is the number of plies, which here is 2 (2 ply). You know what one ply of your yarn is, so divide the total of the yards above the line by the number of plies. 
Simple! Unfortunately, some yarn makers put their fractions upsidedown. Usually, you can consider the smaller number to be the plies and the larger number to be the size. If after your calculations you get a number that makes no sense, try flipping the fraction and recalculating. If you have the yarn in your hands, you can count the plies, so you can tell which part of the fraction refers to it. 
Taken from: Book 1: “Winding a Warp and Using a Paddle”
Thanks, Peggy! This is the clearest explanation of yarn sizes I have seen yet. I will add it to my resource list for my students.
It is also in my book, Weaving for Beginners. It’s what I taught my beginning classes for 10 years or more. Peggy
thank you..a very experienced knitter on knitting paradise led me here when i asked a question of what does 14/2 mean on the inside of a giant cone of cotton rayon yarn..my figure comes out too 5880 whopping yards of yarn..i took 14 divided it by the ply=(7) and then took 840 x 7=5880..Im not a weaver,machine knitter or a spinner..Im a knitter of only a little over 2 yrs and I just could not say No when i bought this giant cone of cotton rayon for $19 bucks. since my yarn is not just cotton but has rayon also would you say my figures are close on yardage?
carolyne
If you have a pound of yarn, you do have that many yards! Good for you! My mentor explained that thinner yarns are cheaper than fatter ones because there are more yards per pound. Peggy
I used to use two, three, or 4 or more strands as one yarn when I got a big cone of thinner stuff than I wanted. Balling or spooling it takes time, of course. Peggy
Hi Peggy, thanks for your explanation. I’m still just learning about all this. 🙂 If I have some linen thread that is 30/3 and some other linen thread that is 30/2, will they be the same thickness? I do understand that the second number means the number of plies. Is the 30/3 made up of 3 thinner plies to make the same thickness as the 2 plies that make up the 30/2? OR do each have the same thickness of ply (as denoted by the 30) so therefore the 30/3 is 50% thicker than the 30/2? So I guess what I am asking is does the 30 refer to the individual ply or the thickness of the combined plies? Thanks so much!
The 30 refers to the size of the ply. Therefore 30/2 (2 plies) is smaller than 30/3 (3 plies). Yarn counts are described in my book, Weaving for Beginners. Grist refers to the size of yarns and is described in y book as well.
Thanks Peggy. Very much appreciated!
Hi Peggy,
Thanks for this explanation. It’s the most helpful one I’ve found online yet. I’m a beginning machine knitter and finding it very difficult to decipher the weight of the cones of yarn I’m trying to buy online without seeing the yarn itself. Is there an easy way to relate the fractions to the weight definitions that hand knitters are familiar with fingering, sport, DK, worsted, ect.? It seems that the numbers are all a jumble and I can’t spot a pattern to them to clue me in to the weight description.
I had a hard time with knitters’ numbers, too and gave up trying to make comparisons. Seems yarns per pound is easier for me but knitters are used to other ways.
I am am Australian and fairly new to weaving. So far, I have used patterns which indicated what yarn and what size reed to use. I use both a rigid heddle and a floor loom.
I would like to now weave without using a pattern. How do I decide what size reed to use? For example, if I use 8/2 cotton, can I use a twelve dent reed, or do I need to double up the cotton for a balanced pattern?
If I use what in Australia is known as worsted, do I use an eight dent reed?
Our yarns are also metric just to make life more difficult and in many cases, the UK system of measurement is used, ie Aran, worsted, etc.
So it’s putting the dimensions of the yarn with the size of the reed which is really confusing me.
For weavers, most everything is based on yards per pound. If you know that you can then decide on your ends per inch and from that the reed. (It’s highly desirable to have two ends per dent.) Other factors can come into the calculation of the sett (epi) eg. weave structure, slippery threads, etc. etc. I think it would be a good idea for you to get my book, Weaving for Beginners”. I have a lot of information on this subject as well as getting started and a series of weaving things to try as you progress. I made this book for you. Since the shipping to Australia is so expensive, I’ll enclose my book #1: Winding a Warp & Using a Paddle” as a gift. In it is a whole chapter on the subject (sett). I hope I’ll hear from you since you are getting started. Anyhow, you work with yards per pound. Hope this helps. Peggy PS You can order on my website–be sure to remind me to include the extra book. You can always email me with the reminder as well: peggy@peggyosterkamp.com.
Thank you for posting this tip. I have been weaving for many, many years and never fully understood how the numbers came into being. I understood the number for plys and that the smaller the number on top, the fatter the yarn (and thus, less yards per pound) but I didn’t know the background for how that number is determined. This information comes at a perfect time for me. We’re doing a series of STEAM based learning programs in my library and I was thrilled to have the opportunity to introduce our teen patrons to weaving, while also showing them that there is a practical reason to pay attention in math class. I will be adding your information to my program. Thanks!
This would be a great application of a math problem–so convoluted but a reason for it. Peggy