The right size sewing thread can make a big difference in your project. A thread that's too thick can weight down delicate fabrics and may not pass through your sewing machine, while one that's too thin may not have enough strength for sturdy seams and heavy fabrics. Unfortunately, many sizing methods exist, several of which you may find difficult to understand. Sizing your sewing thread correctly takes a little research and concentration.
Determine the appropriate thread weight for your project. You don't have to come up with an exact number at this point. Think about the type of fabric you plan to use use and what you'll use it for. Heavy fabrics, such as denim, require a heavier thread. Gauze and satin require lighter thread.
Read the label, if the spool still has one, and check the size printed on it. Most sizing methods measure the weight of a given amount of thread. A spool of regular sewing thread might read “50/2” in the cotton-count method, "502" in the Hong Kong system, "90/2" in the metric system, or "T-21" in the Tex system.
Compare the thread with other spools. If you have old thread, or spools that have lost their labels, you may not be able to read the size. In this case, compare the unknown size with spools with an intact label. Use a magnifying glass or jeweler's loupe to get a better idea of how the threads compare. Though not exact, this method will keep you from using button thread in your sewing machine.
Use a chart. If you have a spool of thread in an unfamiliar size system, a thread conversion chart can tell you the size equivalent in a more familiar system. (See the Resources section below for an example of a thread chart.)
Ask an expert. If you've checked all your other resources and still don't know whether a spool of thread will work for your project, contact someone with experience in this kind of sewing. Workers at your local fabric store may be able to tell you the best thread size.
Sew a test swatch. Cut two small pieces of the fabric you wish to use for your project, and test the thread by sewing them together. If the seam goes together smoothly, you have an appropriately sized thread.
Always use high-quality thread to prevent snarls and tangles.
Take fiber content into account —some fibers are stronger or smoother than others.
No one thread will work perfectly for every project.