The Thread Exchange
Internet-based Thread Sellers
291-A Merrimon Avenue, Weaverville, NC 28787

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Kevlar Thread Information



  About_Kevlar_Thread About_Sizes Nominal_Weights  
  Kevlar_Properties Size_Selection_Guide Certificates_of_Compliance  
  Alternatives_To_Kevlar_Thread Kevlar_Terminology No_Medical_Devices  
  Color How_to_Save Kevlar_Specifications  





About Kevlar Thread--Kevlar™ (and generic equivalents called Para-Aramids) is one of the strongest and most heat resistant commercially available threads. It is about 2½ times stronger than nylon and polyester, has very little stretch, do not melt, and decomposes at 800ºF. This makes it a great choice for high stress applications including sewing seams on heavy duty clothing, conveyer belts, harnesses and bags, reinforcing plastics, and other fibers, and even providing high-temperature controls for model airplanes and rockets. To put Kevlar thread in perspective, here is a side-by-side comparison of Kevlar and three other medium weight (Size 92) threads:






Tensile Strength (Pounds)





Approximate Stretch Factor





Heat Decomposition (Fahrenheit)





Kevlar Properties


Begins to decompose (turn to ash) at about 800 degrees (426 degrees C), in air by oxidation


Does not melt.



Effects of acids and alkalis

Good resistance to dilute acids and bases. Degraded by strong mineral acids and to lesser extent, by strong mineral bases.

Effects of bleaching and solvents

Should not be bleached. Excellent solvent resistance.

Resistance to mildew, aging, and sunlight

Excellent resistance to mildew and aging. Prolonged exposure to sunlight causes deterioration.

Abrasion resistance

Good abrasion resistance.

Alternatives To Kevlar Thread--Kevlar thread is super-strong, fire retardant, and does not stretch. But, it is only available in ugly yellow and is very expensive. Here are five alternatives to Kevlar that might be better suited to your needs.

Nylon and Polyester Thread - Use a heavier weight nylon or polyester thread instead of a lighter weight Kevlar. For example, a Size 138 nylon or polyester thread has about the same strength as a Size 69 Kevlar thread, costs about 80% less, and comes in colors. But, it is twice as thick, not fire retardant, and has 25% elasticity.

Fire Retardant Sewing Thread - Spun Kevlar and spun Nomex threads have the same fire retardance as Kevlar thread and are available in many colors. This type of thread is the usually the best choice for sewing seams and attaching emblems and logos. These threads look and feel like cotton because they are made by wrapping cotton around a Kevlar or Nomex filament core

Fire Retardant Embroidery Thread - Use Nomex embroidery thread to embroider logos and emblems for fire retardant gear. The thread looks like top-quality, 40 weight, polyester embroidery thread, but it is just as fire retardant as Kevlar thread. It is not strong enough to sew seams or attach emblems. It is very expensive and should only be used when fire retardance is essential.

Filament Nomex Thread - This thread has about the same fire retardance as filament Kevlar but only 1/3 of its strength. It has a smooth, shiny feel. We are discontinuing this thread because reliable supplies are not available because of government priorities.

Heavy Duty Twine - Consider using heavy duty twine for projects that require brute strength, a low cost, and new sewing. Our #4 cotton twine has a 385 Lb tensile strength and a 70/1000 inch diameter. It is great for binding, reinforcing, wrapping, and tying. It is not for machine or hand sewing.

Color--Nobody buys filament Kevlar thread for its color. It comes in only one color called Natural (yellow) and it cannot be dyed. Usually, this is not a problem because color takes a back seat to strength and heat resistance in most Kevlar applications.

Color is very important when it comes to fire retardant garments, turnout gear, and racing suits. We have a great selection of spun Kevlar and Nomex™ on our Fire Retardant Thread  page. Spun threads have a single Kevlar or Nomex filament that is wrapped in cotton. This allows dying, retains the thread's fire retardant properties, but sacrifices thread strength. We also have Nomex Fire Retardant Embroidery Thread for names and logos.

Sizes--With Kevlar, a higher size number means a thicker thread. As size increases diameter, strength, needle size, and stitch visibility, and sewing machine requirements increase while yards per pound decreases. Let's see how this affects three representative sizes:

As size increases:

Size 69

Size 138

Size 346

Diameter increases

0.0081 Inches

0.0140 Inches

0.0255 Inches

Strength increases

23 Lbs

45 Lbs

135 Lbs

Needle size increases

16 to 18

20 to 22

26 to 28

Stitch visibility increases



Very High

Sewing machine requirements increase



Heavy Duty

Yards per pound decrease

6,700 Yards

3,350 Yards

1.050 Yards

Size Selection Guide--We sell Kevlar thread in ten standard sizes. Work backwards from the thread strength needed to the thread size that best meets the need using the Kevlar Specifications at the bottom of this page. Keep these points in mind:

  • In many sewing applications the strength and thickness of the materials determines thread size. If the thread is too thick its larger holes will weaken the materials.

  • Size 69 is the thickest thread that most home sewing machines can handle. Use double-stitching to get additional strength.

  • Thinner threads are easier to use in reinforcing and wrapping applications. But, extra strands of thread or double-wrapping may be needed.

Our product listings show two size numbers like this Size 138 (Tex 120). The first number is the thread's U.S. Commercial Size that lets us group products with similar sizes; the second is a more precise Tex size. Most thread is made by twisting plies of Kevlar to make a given size thread. A thread's Tex Size is calculated by multiplying the number of plies by each ply's denier and dividing the result by 10. For example a thread that has four plies of 400 denier thread is a Tex 160 ( (4 plies x 400 denier) / 10 = Tex 160).

We have a mix of standard sizes and  non-standard sizes. The standard sizes meet the specifications shown at the bottom of this page. We sell non-standard sizes at a 50% discount because specifications for these sizes is not available. We try to estimate their specifications using known, standard sizes as the base. We cannot guaranty these estimates.

Kevlar Terminology--We use several "technical" terms to describe our Kevlar thread. Understanding these terms is important because they affect how you can use the thread.


A single ply of Kevlar thread consists of many hair thin filaments of nylon bundled or twisted together.

Left Twist / Right Twist

Most threads are made by twisting multiple plies to make the required size. Left Twist (or Z-Twist) thread is twisted to the left and works with almost all single-needle machines. Right Twist (or S-Twist) thread is twisted to the right and is used with some U.S. double-needle machines and single-needle machine needle machines in Asia. Twist does not matter in hand-sewing, tying, or reinforcing.


The thread has a coating that reduces heat at the tip of the needle and prevents unraveling. Bonding is recommended for machine sewing and is not required for hand sewing. Bonding may interfere with some adhesives.


The thread is not coated. It is great for hand sewing and non-sewing applications. It is not recommended for machine sewing


A monocord normally has many hair-thin filaments, but is only one ply. That is why it’s called a monocord, and not a monofilament.  It feels smooth and is slightly flat or ribbon-like, and tends to be shiny. It is also suited for most double needle machines.

Right Twist

Most thread has two or more filaments that are twisted together. Normally, thread has a left ("Z") twist and we do not mention it. We flag right ("S") twist thread because it will not work in U.S. single-needle sewing machines. It is great for hand sewing and non-sewing applications.

Loose Twist / No Twist

The thread filaments have little or no twist and are frayed. This type of thread is not recommended for any type of sewing. It is a great choice for non-sewing tasks such as reinforcing other materials, netting, and tying


Yarn consists of many hair-thin Kevlar fibers that have not been twisted into a thread. It is not recommended for any type of sewing. It is frequently used to reinforce other materials. If you have patience, you can separate the individual fibers to get a single hair-thin thread that is exceptionally strong for its thickness. Dedicated model makers and prototype designers find this very helpful

How to Save--Kevlar costs about six times more than nylon or polyester. Here are four ways to save:

  • Do you really need Kevlar?--Kevlar seems to have a cult following and customers buy it when less expensive nylon or polyester will do the job. Please, ask yourself or ask us if you really need Kevlar thread.

  • Buy what you need--Our junior spools in 1 to 4 Ounces, coils, and thread packs are a great value for specific, one-time jobs and hobbyists. They generally ship 1st Class to U.S. addresses so shipping costs are low compared to price.

  • Choose lighter weights--You get more yards per ounce with lighter weight thread. But, make sure that the thread has the strength you need.

  • Consider clearance items--We have many clearance items that we are selling at about 50% of our standard thread prices. This thread is older stock that we got when we purchased The Thread Exchange in 2005. It has been stored correctly but we cannot guaranty that it meets the specifications for our standard thread. It is sold "as is" and we do not accept returns or exchanges. Here are some things that you should know about clearance items: 

  • Right twist and soft threads are great for hand sewing and non-sewing applications. They are not recommended for machine sewing.

  • Odd sizes are great values if you have flexibility in thread thickness. These threads are identified by an x at the end of the product code (e.g. KEV138NATL16x4).  An odd sized bonded, left twist thread has the same machine sewing properties as a standard thread. We have estimated odd size thread specifications based on specifications for standard size threads. Please use these estimates as guides with the understanding that they are not guaranteed.

  • Loose twist, no twist, and yarns are great for non-sewing applications such as tying and reinforcing. They are not recommended for machine or hand sewing.

Nominal Weights--Industry norms allow Kevlar thread spool weights to vary by ±10% from the stated nominal weight. We follow this practice, but have biased our standards so that customers receive slightly more thread than less thread in most cases. The table on the right shows the range of weights, including the spool for each nominal spool weight. When spools fall within these ranges we do not charge for overages or refund underages.

Weight In Ounces

Nominal Minimum Maximum



















Certificates of Compliance--We provide manufacturer's CERTS for our regular, standard size Kevlar thread on request. CERTS apply to individual batch / lots. We cannot give CERTS for "Junior" spools that weigh less than 16 Oz because we wind them in-house and do not track batch / lots. In these cases we can send a representative CERT stating that the thread purchased might not come from the batch / lot on the CERT. Also, we can custom-wind "juniors" with batch / lot tracking for a $2.00 a spool up-charge. 

No Medical Devices--Our Kevlar thread is clean in the ordinary sense. But, it is not sterile and using it in any medical device could put lives at risk. We will not sell Kevlar thread for use in medical devices.

Kevlar Specifications






15 23 46 69 92 138 207 346 415 693 800


16 21 40 60 80 120 210 400 500 700 800


A A B E F FF 3 Cord 5 Cord 6 Cord N/A N/A



Tensile strength (Lbs)

4 6 14 23 30 45 64 135 150 175 225

Yards per pound

28,000 25,000 10,000 6,700 5,000 3,350 2,100 1,050 900 780 500

Diameter (Inches)

4/1000 6/1000 8/1000 10/1000 11/1000 14/1000 18/1000 26/1000 27/1000 33/1000 45/1000



Tensile strength (kg)

1.8 2.7 6.3 10.4 13.6 20.3 29.0 61.2 68.0 79.2 102.9

Diameter (mm)












The Thread Exchange
291A Merrimon Avenue Weaverville, NC 28787