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Industrial Sewing Thread Strength: Easy Way to Estimate and 4 Methods of Testing

An industrial sewing thread’s strength is a crucial factor for seam strength. Threads with low strength can easily tear during the sewing process if the stress is too high (when the thread is being tensioned too strongly during the stitch formation). The term “strength” when talking about thread usually refers to the maximum tensile strength (measured in cN). The maximum tensile strength is the maximum linear tensile strength a thread can tolerate before it breaks. Depending on the raw material, thread type, and fineness (linear density), threads can have very different strength levels.

Cotton threads have lower strength than polyester threads, for instance, due to the raw material. Bulk and spun threads have lower strength than core spun threads or continuous filaments due to their construction.

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Easy Way to Estimate Sewing Seam Strength

You will need to know a few things before you get started. You need to know the stitch type (301 lockstitches or 401 chain stitch), stitches per inch (SPI), and thread strength which is usually measured in pounds. A seam is only as strong as its weakest link, so it is important that you are using the same size threads for both your needle (top thread) and looper (or bobbin) when using the following calculations to get the most accurate estimate.

Estimated Seam Strength Formulas

  • 401 Chainstitch Formula: Stitches per inch x thread strength x 1.7 = seam strength
  • 301 Lockstitch Formula: Stitches per inch x thread strength x 1.5 = seam strength

An example of this using a Bonded 207 Nylon with 35 LB Tensile strength, with a 401 Chainstitch, would be calculated as, 5 SPI x 35 LB x 1.7, which would equate to an approximate seam strength of 297.5 lbs.

That is how you can calculate your seams’ strength.

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4 common methods of testing strength

Measurement of strength and elongation is very important because the loop properties, the most important characteristics of a sewing thread, are strongly dependent on tensile strength and elongation. Also, the performance of the seam is very much dependent on the tensile strength and elongation properties of a sewing thread.

Tensile Strength

This is determined by how much force (in grams or kg) is needed to break the thread. A higher tensile strength would indicate a stronger thread because more weight is needed to break it. Good tensile strength to grip the seam firmly during wear and wash. The tensile strength should be higher than that of the fabric so that it won’t rupture during the stretch at the seams. Also, it would mean minimum thread breakage during sewing.


This is determined by the maximum tensile force divided by linear density. The takeaway here is that you want the thread tenacity to match the fabric strength or be slightly weaker. The exception is performance products. The thread should always be just as strong or stronger than the fabric. The loop elongation of a sewing thread is also one important parameter contributing to the elongation of a seam, along with the stitch and seam type, the number of stitches per inch, and the nature of the material stitched.

Loop Strength

This is tested by looping a thread strand through a loop of the same thread and breaking it. This measurement indicates the brittleness of the thread and overall stitch strength.

The loop-breaking strength is measured to assess the ability of a sewing thread to contribute to seam performance. It bears a direct relationship to stitch-breaking strength and hence to seam-breaking strength.

Knot Strength

This term indicated how much load the thread can withstand when knotted before breaking. The knot strength of a sewing thread is considered a measure of the brittleness of the thread. Reduction in knot strength reflects the performance of a thread after stitching.

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Works Cited: Easy Way to Estimate Sewing Seam Strength, THE ULTIMATE GUIDE TO UNDERSTANDING SEWING THREAD