Is bonded nylon stronger than polyester?

Nylon has slightly higher tensile strength and better abrasion resistance at the same size. Polyester has far better resistance to ultraviolet light. For anything that sees sunlight — marine, motorcycle, garden or campervan work — polyester is the correct choice despite nylon being marginally stronger, because UV degradation will destroy a nylon seam long before it wears out.

What thread should I use for car seats?

Bonded nylon in T70 to T90. That is around the range vehicle manufacturers use: strong enough for the seat, with a stitch that stays discreet. Nylon’s abrasion resistance suits a seat that is slid across every day. For decorative top stitching, T135.

Can a domestic sewing machine handle upholstery thread?

Up to T70, usually yes — that is the recognised ceiling for domestic machines and enough for much furniture work. T90 and above need an industrial machine: a larger needle, a bigger hook and more thread path than a domestic head provides.

Upholstery thread guide — sizes, nylon vs polyester

Q: What is bonded thread?

Thread given a resin coating after twisting. The coating binds the plies so the thread resists untwisting as it runs through the machine, reduces friction at the needle and stops the end fraying. Unbonded thread of the same size will fray and snarl at upholstery speeds.

Which thread for which job, what the three numbering systems mean, and the one property that decides nylon or polyester.

If you read nothing else: use bonded polyester for anything that will see sunlight — marine, motorcycle, garden, campervan, caravan — and bonded nylon for indoor work and vehicle interiors. T70 is the everyday upholstery size. Most people who get this wrong put nylon somewhere sunny.

The three numbering systems

The same thread is sold under three sets of numbers, which is most of why this is confusing. Commercial sizes (#69, #92) are the American trade standard. Tex is the international one and is the weight in grams of 1,000 metres — so a higher number is a thicker thread. Government sizes (E, F, FF) appear on a lot of US-made thread and are a legacy of military specification.

They line up like this. Needle sizes are given as ranges because that is how the thread makers publish them.

Commercial	Tex	Govt.	Needle	What it is for
#33	T35	A	80/12 – 90/14	Very light work, linings, fine detail. Rare in upholstery.
#46	T45	B	90/14 – 100/16	Light upholstery, thin leather, cushion interiors, delicate work.
#69	T70	E	100/16 – 110/18	The standard upholstery thread. Sofas, chairs, most seams. The heaviest most domestic machines will manage.
#92	T90	F	110/18 – 125/20	Automotive and vehicle trim, vinyl seating, heavier furniture. Needs an industrial machine.
#138	T135	FF	125/20 – 140/22	Visible top stitching, leather, decorative seams where the stitch is meant to be seen.
#207	T210	3-cord	140/22 – 180/24	Heavy leather, bold contrast stitching, hard-use seating.
#277	T270	4-cord	180/24 – 220/26	Saddlery, very heavy leather, deliberate feature stitching.
#346	T350	5-cord	220/26 – 260/28	Harness work. Beyond most upholstery machines.

T70 is highlighted because it is the size you will use most, and the heaviest a domestic machine will generally manage.

Nylon or polyester

Both are bonded, both are strong, and for most indoor work either will do. The difference that actually decides it is ultraviolet light.

Bonded nylon

Best for: Indoor work, vehicle interiors, leather

In its favour

Highest tensile strength for its size
Excellent abrasion resistance — suits seams that get slid across
More elastic, so it gives rather than snapping under a sudden load
Widest colour range in most suppliers’ stock

Against it

Degrades faster in ultraviolet light
Absorbs a little moisture, which matters in damp conditions

Bonded polyester

Best for: Anything that sees sunlight

In its favour

Far better UV resistance — the deciding property outdoors
Low moisture absorption
Resists most cleaning chemicals well
Nearly as strong as nylon at the same size

Against it

Slightly less abrasion resistant than nylon
Slightly less elastic

The mistake worth avoiding. Nylon is the stronger, more abrasion-resistant thread, so it feels like the better choice — and people reach for it out of habit. Put it in a boat, a motorcycle seat or a campervan and the sun will have it long before the seam wears out. In sunlight, polyester wins on the only property that matters there.

What is bonded thread?

A resin coating applied after twisting. It binds the plies together so the thread resists untwisting as it runs through the machine, reduces friction at the needle, and stops the end fraying when you thread up. For any upholstery work at speed, bonded is what you want — unbonded thread of the same size will fray and snarl.

Which thread for what

Job	Thread	Size	Why
Sofas and armchairs	Bonded nylon or polyester	T70	The everyday upholstery size. Strong enough for any domestic seam without the stitch becoming a feature.
Dining chairs, cushions, light work	Bonded nylon or polyester	T45 – T70	T45 where the seam is short and the cloth light; T70 if in doubt.
Car and van seats	Bonded nylon	T70 – T90	Around the sizes vehicle manufacturers use — strong, and the stitch stays discreet. Nylon’s abrasion resistance suits a seat that is slid across daily.
Decorative top stitching on leather	Bonded nylon or polyester	T135	Bold enough to read as a deliberate line rather than a seam.
Vinyl seating	Bonded polyester	T90	Vinyl is usually a hard-use or wipe-clean situation. Polyester copes better with light and cleaning products.
Leather furniture	Bonded nylon or polyester	T90	T135 where you want the stitching seen.
Marine and boat upholstery	Bonded polyester	T90 – T135	Polyester, always. Sunlight is the thing that kills a marine seam, and nylon degrades in UV faster than polyester.
Motorcycle seats	Bonded polyester	T90 – T135	Outdoors and uncovered most of its life. Same reasoning as marine.
Garden and outdoor furniture	Bonded polyester	T70 – T90	Polyester for the UV. This is the one choice people get wrong most often.
Campervan interiors	Bonded polyester	T70 – T90	Behind glass, in the sun, for years. A van interior gets far more light than a sofa in a front room.
Caravan and holiday-home seating	Bonded polyester	T70	Hard use, strong light, and cleaned often.

S twist and Z twist — and why the trade contradicts itself

Plied thread is twisted in one of two directions, described as S or Z. Sewing machines are built around one of them, and using the wrong one makes the thread untwist as it runs, which shows up as fraying, snarling and broken thread that no amount of tension adjustment will fix.

Machine sewing thread should have a final Z twist. That is true for domestic, industrial and longarm machines alike, and for every major brand. S twist is for hand sewing, and for the outside needle of certain twin-needle machines built for it.

Ignore “left twist” and “right twist”. The trade uses those terms inconsistently. One large supplier’s thread guide calls Z twist the left twist while their own blog describes S twist as clockwise; another supplier reverses the pair entirely. The S and Z labels are unambiguous and the left/right ones are not, so buy on S or Z and disregard the rest.

Can a domestic machine sew upholstery thread?

Up to T70, usually yes — that is the recognised ceiling for domestic machines, and it is enough for a great deal of furniture work. Above it you need an industrial machine: T90 and up want a larger needle, a bigger hook and more thread path than a domestic head is built for. Forcing it produces skipped stitches and, eventually, a timing problem.

Needle sizing has its own page: choosing the right needle.

Sources. Every figure on this page is corroborated across at least two of these:

Last checked 2026-08-02. Thread and needle sizing is standardised, so this does not drift the way fire regulations do — but manufacturers vary slightly, and your thread supplier’s own chart beats any general table including this one.

Upholstery thread guide