Kevlar Thread Buying Guide

Kevlar Thread Buying Guide

About Kevlar Thread  Kevlar™ thread (also called Para-Aramid) 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, does 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 middleweight (Size 92) threads:








  Tensile strength

30 Lbs.

15 Lbs.

15 Lbs.

9 Lbs.

  Stretch before breaking





  Heat decomposition






Kevlar Properties Kevlar thread is stronger, more heat resistant, less stretchy, durable than most other threads. But, thread is usually a very small part of material content. This means that using Kevlar thread with materials that do not have its properties is a waste and could be dangerous.


Kevlar begins to decompose (turn to ash) at about 800ºF  (426ºC). Much higher than nylon or polyester.


Kevlar thread does not melt. This is important in fire retardant gear.


Kevlar thread stretches about 2% before it breaks. For example, a 100 foot piece of Kevlar would be 102 feet long just before it breaks. If you sew stretchy fabrics with Kevlar the seams will pucker.

  Acids and alkalis

Good resistance to dilute acids and bases. Degraded by strong mineral acids and to lesser extent, by strong mineral bases. This is important when Kevlar thread is used in a laboratory setting

  Bleaching and solvents

Should not be bleached. Excellent solvent resistance. This matters when Kevlar gear is reused.

  Mildew, aging, and sunlight

Excellent resistance to mildew and aging. Prolonged exposure to sunlight causes deterioration.  Polyester thread works just as well in some cases.


Good abrasion resistance.


Alternatives To Kevlar Thread--Kevlar thread is super-strong, fire retardant, and very little stretch. But, it is only available in ugly yellow and is very expensive. Here are 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 reliable because of government priorities.


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

There is some black filament Kevlar on the market. But, we do not normally stock it because we have not found a reliable source and there are alternatives.

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 thread, 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 In most sewing applications thread is the least expensive material. This means that it is often (parachutes excepted) better for the thread to break than for the fabric to rip. It also means that fabric and leather thickness and strength is the most important factor in selecting the right size. Simply put, thread size should normally increase as material thickness and strength increase. There are two other considerations:

  • Most home sewing machines cannot handle threads heavier than Size 69. In these cases use double stitching or box stitches to add strength.
  • Sometimes "look" is important. Using a heavier thread makes stitches stand out from the fabric or leather.

Size selection works differently in reinforcing, wrapping, hanging, and whipping applications. Here, the other materials are not as strong as Kevlar and the strength of the thread is often key. There are exceptions such as threading Kevlar through a small hole.

Take availability into consideration when there is an ongoing need. We regularly stock Kevlar thread in ten sizes and do our best to keep them in stock. We also have thread in sizes that we do not regularly stock and sell them at lower prices because we will probably not have them when our current supply runs out.

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.


Bonded / Soft

Bonded means that the thread has a coating that reduces heat at the tip of the needle and prevents unraveling. Soft means that the thread does not have this coating.

Bonding is important for machine sewing in Sizes 69 and higher. It does not matter when the thread is used for hand sewing and reinforcing. Bonding may interfere with adhesives used in some wrapping and whipping applications. If so, it can be removed with alcohol.



Monocord means that the thread is made with a single ply (like fishing line) and has a coating similar to bonding. This gives the thread a flat, ribbon-like shape and makes it usable in double-needle machines. Many of our lighter weight threads use this construction


Left twist / Right twist

When threads have more than one ply the plies are twisted to the left or the right. Unless we say otherwise our thread is left twist. This is because single-needle machines made for the U.S. market require left twist or monocord thread.

Use right twist thread with double-needle machines and some machines designed for Asian markets. Twist does not matter in hand sewing and non-sewing applications.


Loose twist / No twist / and Yarn

These kinds of threads are frizzy and almost impossible to put through a needle. Consider them for non-sewing applications such as wrapping and hanging.


How to Save--We love selling Kevlar thread. But, it costs about five 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 non-standard sizes and closeouts These threads sell at a 25% to 50% discount compared to our regular thread. Our featured Tex 105 and Tex 300 Kevlar thread is on par with our standard sizes. But, we cannot provide certificates of conformance (CERTS) and will probably not restock them in the future. Our closeouts are old thread that sells at very low prices, but is not recommended for machine 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. Here is a table that shows our tolerances. When spool weights fall within these tolerances we do not charge for overages or give refunds for underages.


Nominal Weight

Spool Minimum

Spool Maximum

Nominal Weight

Spool Minimum

Spool Maximum



2.0 Oz

2.6 Oz

4.5 Oz

8.0 Oz

8.0 Oz

10.5 Oz


4.0 Oz

4.6 Oz

5.9 Oz

12.0 Oz

10.6 Oz

15.5 Oz


6.0 Oz

6.0 Oz

7.9 Oz

16.0 Oz

15.6 Oz

19.9 Oz


Certificates of Compliance We can email manufacturer's CERTS for our 16 Ounce standard size Kevlar. We cannot provide CERTS for any non-standard size or closeout thread.

CERTS apply to specific batch / lots. We wind our smaller spools from the 16 Oz standard size spools and do not keep batch / lot information for these spools. We can send a "representative" CERT for these spools that applies in a general way to the thread purchased. Please call if you must have batch / lot traceable CERTS on small spools and are willing to pay a $2 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 knowingly sell Kevlar thread for use in medical devices.

Kevlar Specifications Here are specifications for our standard size Kevlar thread














































Tensile Strength (Lbs.)











Yards Per Pound











Diameter (Inches)






















Tensile Strength (Kg)











Diameter (MM)











Non Standard - No CERTS Available

Metal Reinforced Kevlar

Tex 16 / Monocord

Tex 105 / Soft

Tex 300 / Yellow

Tex 300 - Black

Half-Price Kevlar

Magicians Invisible Thread

Kevlar thread with a hair-thin metal core for better abrasion resistance. Approximately Size 92 / Tex 80. Strength is unknown.

Hair-thin, single filament thread used for fine sewing and reinforcing tiny objects.

This thread comes on 1-pound tubes, is about 15% thicker than Size 92, has 40 pound strength, and is soft.

Tex 300 bonded Kevlar or 10% heavier than U.S. Size 277. Tensile strength is 84 pounds. 25% off sizes we regularly stock.

Hard to find Tex 300, bonded, black, filament Kevlar. About 10% thicker than U.S. Size 277 with an 84 pound tensile strength.

Kevlar thread in various sizes priced at half our regular prices. This thread is not recommended for machine sewing.

Black Kevlar yarn that magicians split into hair-thin filaments for levitation illusions

Related Products

Fire Retardant Sewing Thread

FR Embroidery Thread

Nomex Thread

Nylon Thread

Polyester Thread

Kevlar and Nomex thread wrapped in cotton. Used for sewing garments. Comes in many colors

Lightweight, fire retardant, Nomex thread used to embroider first responder names and emblems.

Filament Nomex thread in various sizes. Mostly white and some colors. Closeout priced.

Save 80% by using nylon and polyester thread. These threads are strong enough for most sewing tasks especially when you can double-stitch or use thicker sizes. But, they are not fire retardant.

Related Products
Sewing Thread Embroidery Thread and Bobbins Filament Nomex Filament Kevlar

Spun Kevlar and Nomex used for sewing and attaching patches. Feels like cotton

Nomex thread and bobbins that look and sew like embroidery thread. Use for names and emblems.

Closeout. Mostly white and green tint. Alternative to FR sewing thread

Stronger than sewing thread and just as fire retardant. Only comes in yellow


The Thread Exchange 291A Merrimon Ave Weaverville, NC 28787 - 800-915-2320