This article explains the strength characteristics of various types of heavy duty sewing machine thread. We’ll cover tensile strength as well as the pros and cons of each type of sewing thread.
What is the strongest sewing thread? Kevlar is the strongest thread for sewing machines while Nylon is half as strong but much more commonly used. Regular sewing machines should use thread sizes no greater than 69. Beyond that, you’ll need industrial sewing machines and specialized sewing needles.
There are several “lifetime threads” similar to Kevlar that produce thread with incredibly high tensile strength used for sailing repairs, paragliding, military gear, industrial gear, heat resistant clothing, sports gear, etc.
In this list however, we included only the readily available threads that can easily be used in your everyday sewing projects. These include clothing, upholstery, linen, cushions, curtains, beading, sewing buttons and more.
Commercial thread sizes range from 15 to 346 (more info below). All threads on this list are size 69, the maximum for home sewing machines. Also see Best Heavy Duty Sewing Machines.
1. Kevlar
Size 69 breaking strength: 23 pounds (10.43 kg)
Kevlar, made by DuPont, is a heavy duty, heat resistant polymer (Phenylediamine with Terephthaloyl chloride) used in industrial, military and other highly durable products like firefighter gear and even paragliding lines.
It is available only in yellow and is five times stronger than steel (on equal weight basis).
Reebok, Adidas, Supreme and NAPA, have all used Kevlar in their manufacturing and sewing processes.
It has low elasticity and can stretch 3-5% before breaking. It is extremely heat resistant and can survive over 600F. It completely decomposes at 800F (427C) where it will char rather than melt.
Costs around 5 times more than Nylon and is complete overkill for most home sewing projects.
That’s why, Nylon is more often than not, a better choice…
2. Nylon
Size 69 breaking strength: 11.31 pounds (5.13 kg)
Because if it’s ability to stretch before breaking, Nylon is considered the strongest heavy duty, regular thread.
It’s used for automotive interior and indoor upholstery, athletic mats, backpacks, horse blankets, dog leashes, furniture upholstery, outdoor gear, garments, leather work, drapes and curtains.
This heavy duty sewing thread is available in many colors.
The molecules of Nylon are more ordered (like rungs of a ladder) which gives it a pretty high natural strength.
It is also the most elastic option and can stretch 30-40% before breaking.
Unfortunately it’s not very resistant to UV rays and should therefor not be used for things like outdoor furniture and sails.
Deteriorates at 285F (140C) and starts to melt at 485F (251C)
3. Polyester
Size 69 breaking strength: 10.13 pounds (4.59 kg)
Polyester is essentially a type of plastic (polyethylene terephthalate) made from a chemicals reaction between petroleum, water and air. It is highly resistant to UV rays making it a great strong sewing thread for outdoor furniture covers and upholstery.
It is hydrophobic making it pretty much waterproof. Typical uses are outdoor patio cushions that are meant to be completely weather proof, pool covers, banners and flags.
The thread lasts around 10 years with high resistance to UV rays, mildew and salt water. If needed, you can even purchase UV treated polyester for further protection against the sun’s rays.
The melting point is around 482F (295C)
4. Cotton
Cotton is a natural and sustainable material as well as being hypoallergenic. Unfortunately, it is also not very strong.
Its by far the most common sewing thread used on everyday household sewing projects. It is not waterproof and will wear over time with enough friction.
It’s regularly used in all types of clothing, linen, indoor curtains, cushions, soft toys, etc.
The molecules are randomly arranged making it pretty weak.
It also has pretty average elasticity and stretches 8-10% before breaking.
Doesn’t melt but has a low heat resistance. More like Kevlar, it turns to ash when it chars.
Burns/Ignites at 400F (204C)
Heavy Duty Thread Size Guide
There are 3 main types of thread sizing: Commercial, Tex and Government. Here’s how they compare as well as which heavy duty sewing needle is advised.
| Commercial Size | Tex Size | Government Size | Needle Size |
|---|---|---|---|
| 15 | 16 | A | 10-12 |
| 30 | 30 | AA | 12-14 |
| 46 | 45 | B | 14-16 |
| 69 | 70 | E | 16-18 |
| 92 | 90 | F | 18-20 |
| 138 | 135 | FF | 20-22 |
| 207 | 210 | 3-Cord | 22-24 |
| 277 | 270 | 4-Cord | 24-26 |
| 346 | 350 | 5-Cord | 26-28 |
As explained earlier, commercial size 69 is the heaviest thread you should use on home sewing machines.