Nylon wins on abrasion and stretch. Polyester wins on sunlight, colorfastness and everyday machine work. Pick the fiber that survives whatever is going to kill the seam: a strap rubbing on a buckle wants nylon, a cushion that lives on a porch wants polyester, and an ordinary garment seam wants a 40wt polyester because nothing about that job is extreme.
Most comparisons of these two fibers turn into a tensile strength contest, and tensile strength is the least useful number on the spool. Both will outlast the fabric they are sewn into. What separates them is how they behave after a season of use.
Why tensile strength is the wrong scoreboard
A seam rarely fails because one strand was pulled apart in a straight line. It fails at the fabric edge, at the stitch line, or at the thread's surface. Abrasion thins the thread where it crosses a buckle, a zipper pull or the needle eye. Sunlight breaks the polymer while the spool still looks new. Flex fatigue works the same short section back and forth until the fibers split and the surface goes fuzzy.
Those three failure modes do not point to the same fiber.
Where nylon earns its reputation
Nylon's advantage is wear. In the same construction it resists abrasion better than polyester, which is why bonded nylon is the standard in leather goods, auto upholstery, webbing, saddlery and anything that drags across a hard edge. The fiber selection charts used by industrial thread makers say the same thing in one line: abrasion resistance, nylon; UV resistance, polyester. Bonding means the plies are held together with a resin coating that also cuts friction and slows the unraveling at a cut end.
Unbonded multifilament nylon is slicker and livelier, and it frays at a cut end. Bonding trades a little softness for a surface that stays together under a rubbing load, which is what a leather seam needs. On hand work the waxed version has a second benefit: the coating gives the thread enough grip to sit still in a large needle eye instead of sliding out mid-stitch.
The trade-off shows up at the needle. Bonded nylon is sold in heavy sizes, tex 70 and up, and those sizes do not fit the eye, the groove or the tension range of a typical home machine. A domestic machine that sews shirt seams all day will balk at tex 70, and forcing it is how needles get bent and timing drifts. That is a mismatch between thread and machine rather than a defect in the thread, and the compatibility guide covers where the ceiling sits.
Where polyester wins
Polyester's advantage is weather. It holds its strength in sunlight far better than nylon, absorbs almost no water, and keeps its color through repeated washing. That is why outdoor fabricators specify polyester thread for boat covers, awnings and outdoor cushions, and why a nylon seam left on a trailer through a few summers can fail while the fabric around it still looks serviceable.
Nylon's weakness under UV shows up as yellowing as well as strength loss, and it is most obvious on white and cream. On a wallet or an indoor chair, none of that ever appears. On a cover that lives outside, it decides the job.
Stretch helps in one place and hurts in another
Nylon elongates more before it breaks. In webbing, straps and anything that flexes, that give is an asset: the seam moves with the load instead of concentrating it on one stitch. On a seam that is supposed to hold a shape, such as a bag gusset or a waistband, the same give can read as a loose stitch, and no tension adjustment removes all of it because the thread itself is stretching.
Woolly nylon is the extreme case. It is textured and bulky, built for serger loopers where the stitch is designed to extend, and it is the wrong thread for a lockstitch no matter how the tension is set. The knit seam article covers why the stitch pattern, not the spool, is the thing to change.
Heat, irons and the white spool
Both fibers are thermoplastic. A hot iron can glaze either one, and nylon is the one that also yellows, which on a pale garment is permanent. Press from the wrong side or through a cloth, and keep the iron off exposed nylon topstitching on webbing and straps.
Heat matters at the needle too, where friction raises temperature at speed. Thermoplastic thread that gets hot enough will soften, stretch and thin at the eye, then break a few inches later. Needle choice and a sensible speed matter more than the fiber at that point, and the manufacturing walkthrough explains what the factory does to keep that heat from winning.
Do not compare a heavy nylon to a fine polyester
Labels make this harder than it needs to be. Heavy nylon is usually sold by tex or by a ticket number, polyester by weight, and a spool that says only "heavy duty" tells you nothing about either. When the size is missing, compare a strand of each against a needle eye and trust the one that passes without crowding it.
Most online arguments about these fibers compare a tex 70 bonded nylon with a 40wt polyester and conclude that nylon is stronger. Of course it is: the nylon runs between one and a half and twice the diameter of the polyester. Fiber and size are two separate variables, and the spool label usually gives you only one of them. A 40wt nylon and a 40wt polyester are close enough in tensile strength that the difference rarely decides anything; what changes is how each one wears and how each one ages.
That is why the useful question is not "which is stronger" but "which one is still doing its job in three years."
What this means on a home machine
Almost everything sewn on a domestic machine is 40wt polyester work: garment seams, quilting, topstitching, general mending. It is fine enough for a 75/11 needle, strong enough for denim, and stable enough that tension behaves. The needle guide explains how to match the eye to that weight when a project moves up to canvas or multiple layers.
Nylon belongs to the heavier jobs, which usually means hand repair and heavy machine work on leather, canvas, upholstery, bags and shoes. That split is also a color decision. Heavy nylon comes in a short list of colors because it is sold for repair, where black, white and brown cover most of the work. Polyester cones come in a wider range because garment sewing is a color-matching exercise.
A 60-second decision
| If the seam will | Choose | Because |
|---|---|---|
| Rub on hardware, webbing or a hard edge | Nylon | Abrasion resistance and flex |
| Sit outdoors in sun and weather | Polyester | UV stability and low moisture uptake |
| Be washed often and keep its color | Polyester | Colorfastness and low absorbency |
| Run through a home machine | 40wt polyester | Fits the eye, groove and tension range |
When the answer is still close, choose polyester. It is the safer default for anything that will be washed, worn and left in daylight, and the failure it protects against is the one that takes longest to notice.
Buying one or the other
The heavy-duty nylon set is the repair side of this: 4-ply tight-twist nylon with a wax infusion, three colors, and the needles that heavy work needs. It is a kit for leather, canvas, upholstery and bags, and those three colors are the limit of it. A specific olive canvas sits outside its range.
The 2,000-yard 40wt polyester cone is the other side: 17 colors, fine enough for a home machine, stable in sunlight, and wash-fast. It will not sew a saddle. Those two limits are the honest summary of the comparison.
One exception is worth naming. When the repair is on worn fabric rather than a strap, heavier is the wrong direction even if it feels more serious, and the mending guide explains why a lighter thread usually outlasts a stronger one there.
