Choosing Between Cotton and Polyester Thread

A seam can be as straight as you like, but if the thread snaps after the first wear, the fabric puckers or the colour suddenly looks out of place, the thread wasn't the right choice. So when it comes to cotton or polyester thread, it's not just a matter of personal preference. The type of fabric, the strain on the seam, the look you're after and the quirks of your sewing machine all play a part.

Because a good sewing thread often looks so unremarkable on the reel, it's easy to forget when you're buying fabric. Yet it's what holds your project together – from a home-sewn T-shirt to children's trousers or a make-up bag. With a few clear criteria, you'll reliably find the thread that suits what you're making.

Cotton or polyester thread: the key differences

Cotton thread is made from natural fibres and usually feels soft with a pleasantly matt finish. It looks lovely with woven fabrics made from cotton, linen or other natural fibres. It's a popular choice for patchwork, quilts and classic sewing projects with a matt surface. It's easy to work with, can be washed at high temperatures and takes colour beautifully.

Polyester thread is a synthetic thread and, in everyday sewing, often the versatile all-rounder. It's strong, slightly stretchy, abrasion-resistant and copes better with frequent washing. That little bit of give is a real advantage when the fabric stretches as you wear it – for example with jersey, sweatshirt fabric or blends. Polyester thread also tends to stay colourfast for a long time and dries quickly.

Neither thread is better across the board. Cotton thread wins when a natural feel and a consistent choice of materials are your priority. Polyester thread comes into its own when a seam has to withstand a lot or the fabric is stretchy.

Stretch often makes the difference

The most common reason for split seams isn't necessarily a poor machine, but a thread that behaves differently from the fabric. A jersey seam stretches every time you put the garment on or take it off. Pure cotton thread barely stretches at all. So if the seam is put under a lot of strain, the thread can snap even though the jersey itself remains perfectly intact.

Polyester thread moves with the fabric much better, which is why it's usually the easier choice for stretchy dressmaking fabrics. With very stretchy fabrics you'll also need a stretch stitch, such as a narrow zigzag or a suitable stretch stitch on your machine. Even the best thread can't make a rigid seam stretchy on its own.

Which thread suits which sewing project?

For clothing made from jersey, French terry, sweatshirt fabric, softshell or many modern blends, good-quality polyester thread is an excellent choice. It stays stable as you move, survives plenty of washes and is ideal for children's clothes that get a lot of wear. For hems, side seams, ribbing edges and pocket bags, the thread can happily be on the hard-wearing side.

If you're sewing a dress, blouse or apron from cotton woven fabric, you can use either type of thread. If you want a particularly natural, matt look, cotton thread works beautifully. If the garment will be washed regularly or needs to stand up to busy family life, polyester thread often lasts longer. So it's not just the fibre of the fabric that matters, but also how the finished piece will be used.

For bags, rucksacks, storage baskets and cushions, polyester thread is usually recommended. Canvas, home decor fabrics and sturdy cottons put a lot of tension on the seams, especially at straps, corners and zips. Here, strong polyester thread gives you a good safety margin. For very thick materials, a heavier thread may also make sense – but do check that your needle and machine can handle it.

In patchwork and quilting, cotton thread is a much-loved classic. It joins cotton fabrics harmoniously, adds little bulk and gives a lovely, understated finish. If your quilt will be used and washed often, though, you're welcome to reach for polyester. Modern polyester threads are fine and can be just as discreet. For visible quilting lines, it mainly comes down to the effect you want: matt and natural, or with a slight sheen and extra durability.

Embroidery has its own rules

Embroidery thread isn't automatically sewing thread. It's made for decorative stitching and is often twisted differently, shinier or thicker. For machine embroidery, you should therefore use special machine embroidery thread. For hand embroidery, cotton, polyester, wool or novelty threads can all work, depending on the technique.

If you just want to stitch on a small embellishment, ordinary sewing thread may be enough. For a larger embroidered area, a monogram or a motif on children's clothing, it's worth using proper embroidery thread: it gives you a more even stitch and is kinder to your machine.

It's not just the fibre: how to spot quality thread

A cheap thread can look fine at first glance and still cause trouble when you sew. If it sheds a lot of lint, snaps for no obvious reason or has uneven patches, your stitching will suffer. You'll need to clean your machine more often, and the seam will be unnecessarily weak.

Look for smooth, evenly twisted thread of good quality. It should unwind cleanly from the reel without knots. For most domestic sewing machines and everyday projects, a medium-weight all-purpose sewing thread is just right. Very fine fabrics need a finer thread, while sturdy bag fabrics or heavily stressed seams sometimes call for a heavier one.

Colour deserves attention too. A tone-on-tone match is the safest choice for clothing and bags. Lay the reel directly on the fabric and look at it in daylight: a shade that seems perfect on the reel can suddenly stand out against a warm beige, a cool grey or a patterned fabric. With prints, it's best to go by a calm background colour or a shade that crops up frequently in the pattern.

Matching needle, tension and thread

Even the right thread won't sew nicely if the needle is blunt, too small or the wrong type. As a starting point, a size 80 universal needle works for many woven fabrics. For jersey and other knits, a jersey or stretch needle is better, as it's less likely to damage the loops of the knit. For heavy canvas, several layers of fabric or bag interfacing, you may need a larger universal or denim needle.

If your top thread keeps snapping, don't reach for the tension dial straight away. First change the needle, rethread both the top thread and the bobbin, and check that the reel can unwind freely. Only then is it worth looking at the tension. Many problems come down to a worn needle or a small threading mistake.

Before starting your actual project, always sew a quick test seam on a scrap. Use all the layers that will be joined later – for example fabric, interfacing and ribbing. Gently tug on the seam, check the underside and, for delicate projects, give the scrap a test wash. Those two minutes often save you a lot of unpicking.

Common mistakes when choosing thread

A common misconception is that cotton fabric must always be sewn with cotton thread. That can work well, but it isn't a hard and fast rule. Sewing a cotton bag with polyester thread makes sense if it's going to be loaded up regularly. Conversely, cotton thread can be exactly the right choice for a decorative seam on a woven fabric that won't be under much strain.

Old thread from your sewing box also deserves a critical look. If it has spent years in sunlight, damp or high heat, it can become brittle. Test it by gently pulling a length apart between your hands. If it snaps very easily, it shouldn't be used for load-bearing seams any more.

Also avoid mixing top and bobbin threads at random. For a normal machine seam, both should ideally be the same type and weight of thread. Different threads can react differently to strain, washing and tension. There are exceptions for special effects, but for everyday sewing a matching combination gives you the neatest result.

A handy decision guide

When you're standing in front of the thread display, these questions will help you choose:

• Is the fabric stretchy, or does the seam need to move a lot? Then polyester thread is usually the right choice.
• Do you want a matt, natural look with cotton, linen or patchwork? Then there's a lot to be said for cotton thread.
• Will the project be washed often, worn by children or put through its paces? Go for strong polyester thread.
• Is the seam meant to be visible and decorative? Choose based on the sheen, colour and surface effect you want.
• Are you sewing through several thick layers? Also check the thread weight, the needle and what your machine can manage.

If you're unsure, by all means take your cue from the fabric – just don't treat it as a fixed rule. A cosy pair of cotton baby trousers that's constantly in the wash, for instance, often benefits from polyester thread. A hand-quilted cushion made from high-quality cotton fabrics, on the other hand, can look especially harmonious with cotton thread.

It's best to bring a fabric scrap, your sewing pattern and your project idea with you when choosing thread. That way you can compare colours and also judge how much strain your seam will really have to take. And if you're torn between two threads, a test seam will often show you faster than any theory which one feels right for your project.