Understanding Thread Weight
- Thread Weight
- Thread Diameter
How is Thread Measured?
Standards for classifying the size of thread are confusing. It would help if the collective sewing and quilting industry would choose a single standard and apply it across the board, but that hasn't happened yet and most likely isn't going to happen in the future. The most commonly used classification in the U.S. is the weight standard. You will see threads labeled as variations of the following: 30 wt., 40 weight, or fifty wt. Outside the U.S., these standards aren't followed or understood. This method of classifying thread is the most popular and used for quilting and sewing, but it is not the most accurate. There is confusion surrounding what exactly makes a 50 wt. thread a 50 wt. thread. We'll do our best to define thread weight in this article.
Not too long ago, there were only three common sizes of thread in the U.S. Thin threads were labeled 50 wt., regular weight threads were labeled 40 wt., and heavy threads were labeled 30 wt. These numbers, 30, 40, and 50, were borrowed from another standard, known as the Gunze Count standard, established by thread factories in Japan. If a thread was labeled as #40 or 40/3 in Japan, it was labeled as a 40 wt. thread in the U.S. Likewise, if a thread was labeled #50 or 50/2 or 50/3 in Japan, it was defined as a 50 wt. thread in the U.S.
The numbers that trail the slash are the number of plies that make the final, stitch-ready thread. All thread, except for monofilament threads like MonoPoly consist of thin strands, called plies. These plies are twisted together to create the thread we use to sew and quilt.
Thread construction diagram
Close up image of thread plies
The problem with classifying all #50 threads as a 50 wt. thread is that a 50/2 thread and a 50/3 thread are different. The first number follows the Gunze Count standard and indicates the thread size. The larger the number, the finer the thread (a 50/2 will be thinner than a 30/2). The second number indicates the number of strands, or plies, twisted together. It is obvious that a 50/3 is heavier than a 50/2 because it has three strands of a size 50 thread twisted together and the 50/2 has only two. The misunderstanding in the U.S. weight system came about because importers started labeling #30 thread as 30 wt., #40 thread as 40 wt., and #50 thread as 50 wt., regardless of the number of plies comprising the thread. That means a 50/2 and a 50/3 thread were both labeled as 50 wt. thread even though one is 50% heavier than the other (a three ply thread is 50% thicker than a two ply thread). As a result, many products that use the weight classification have labels that are inaccurate.
We regularly field questions from customers trying to compare Thread Brand A to Thread Brand B using the weight system, which doesn't account for the number of plies. As a consumer seeing hundreds of competing products on the market, it is difficult to know which labels are accurate and what the numbers really mean. When we travel, we often visit local quilt shops because Mother Superior always like to search for more fabric (just in case she finds one that she doesn't already have) to add to her overflowing fabric stashes. My excitement is to wander over to the thread racks and read the labels. I can always find some threads that are labeled inaccurately. If you see a label with an odd wt. number such as 17 wt. or 19 wt., it is most likely accurate. However, if it is a commonly used weight such as 30, 40, or 50 wt., it may or may not be accurate. I'm not one to point out a problem without proposing a solution, so here is how to deal with inaccurate labels: Ignore the weight number on the label. Choose thread based on the type of fiber, look, feel, and thickness and not by the printed weight size. Trust your eyes and fingers more than the label. You'll get better results and be much happier with your selection. Choose fine threads to blend and medium and heavier threads to show.
View our education article Causes of Thread Shredding.
View our Thread Selection Guide.