Thread sizes can be almost as confusing as dress sizes! Part of the confusion comes because there are different sizing systems used. For most of our threads, rather than put a numerical weight or size on the label, we use a descriptive term such as Machine Quilting, Upholstery, Fine, All-purpose or Heavy.
We would like to try to clear up some of the confusion about thread size. Thread sizes are based either on a fixed weight system or a fixed length system.
Fixed weight systems describe the length of thread needed to weigh a specific amount. In these systems, the higher the number, the finer the thread. Two systems are the Cotton count which is based on 840 hanks =1 lb and Thread weight based on 30 meters = 1 gram.
Fixed Length systems use the weight of a given length of thread. In these systems the higher the number the larger the thread. Two examples are Tex which is the weight in grams of 1000 meters and Denier which is the weight in grams of 9000 meters.
Then there is size… This is an older system that groups threads into fine (Size 60), medium (Size 50) and heavy (Size 20) categories, with again, the higher number being the finer thread.
For an in-depth explanation of thread size, read Unraveling The Mystery of Thread Size by Ken Nelson, Sr. Product Manager-Sewing Thread, Coats & Clark.
This chart gives the Tex and Weight for Coats threads.