Swimwear Stitch Types Explained, With Photos

Swimwear Stitch Types Explained, With Photos

Swimwear stitch types fall into two groups: the stitches sewn into the suit, and the finishes on its edges. There are four swimwear stitch types: overlock, coverstitch, flatlock and zigzag. Edges are bound, clean-finished or bonded. The clean-finished edge is the “seamless” look in many founder reference photos: elastic hidden inside, no topstitch, no bonding. Your swimwear stitching changes how a suit feels, how long it lasts and what you pay per unit. If you’re starting a swimwear brand, this is the part of a sample that’s easiest to approve without really looking.

What stitches are used in swimwear?

Four stitch types show up on swimwear: overlock, coverstitch, flatlock and zigzag. Each one looks different on the front and the back of the fabric. Turning a sample inside out is the fastest way to tell them apart.

In swimwear construction, each stitch has a job. Overlock joins seams and attaches elastic. Coverstitch is a topstitch with some stretch, usually sewn over an overlocked seam. Flatlock joins edges flat in a single layer. Zigzag takes care of small jobs like labels and reinforcement.

Stitches are only half the picture. How the edges are finished is a separate choice, covered further down.

What is an overlock stitch in swimwear?

An overlock stitch joins two layers of fabric and wraps the cut edge in thread, all in one pass. It’s a standard stitch for stretch fabric, not a shortcut: thread maker Coats recommends an overedge stitch for applying elastic to swimsuits.

On the inside of a suit, you’ll see a band of looped threads covering the raw edge. From the outside, you only see the seam line. Factories call this family of stitches overedge, and the two you’ll hear about are 503 and 504.

For elastic, Coats recommends 503 over 504. It gives more seam stretch and uses less thread. The result is a finer seam that’s more comfortable to wear.

Stretch is where an overlock seam succeeds or fails. A high-stretch seam needs enough thread built into it to reach the stretch the fabric requires, without overstressing the thread. Too little, and stitches pop when the seam is stretched. Ball point needles are recommended on knits like swim fabric.

The trade-off is bulk. An overlock seam stacks the seam allowance on one side, so it’s thicker than a flatlock seam. You’ll often see a coverstitch sewn over it.

What to take from this: if a factory proposes overlock seams, that’s normal. Ask how they build in enough stretch at the elastic, and check it yourself at sampling.

What is a coverstitch used for in swimwear?

A coverstitch is a topstitch made with 2 or 3 needles and no bobbin, usually sewn over an overlocked seam, with some stretch. On the front you see 2 or 3 parallel rows of stitching. On the back, there’s a ladder of looped thread.

The trade-off is visibility. A coverstitch shows on the outside of the suit, and over an overlock seam it adds a layer. If your design calls for no stitching on the face of the suit, a coverstitch isn’t the finish you want. Look at the clean-finished edge below instead.

Flatlock vs overlock: what’s the difference?

Flatlock butts two edges together and joins them flat in one layer. Overlock stacks the seam allowance on one side, so it’s thicker. Flatlock has fewer layers to chafe, which is why it shows up mainly in performance goods.

If your line is built for training or long hours in the water, flatlock is worth asking about.

One thing to watch for: a mock flatlock. It’s made on an overlocker and looks similar from the outside. It uses 3 threads instead of 6, and it’s weaker than a true flatlock. If a sample is described as flatlock, ask which kind it is.

When is a zigzag stitch used in swimwear?

A zigzag stitch handles the small jobs: attaching labels and reinforcement stitching. It’s flexible and can be decorative. Joining the main seams is overlock and flatlock work.

You’ll usually find it in three places on a suit:

• Labels. Care and brand labels sewn inside the suit.
• Stress points. Short, dense rows of stitching called bartacks, which lock a spot that takes a lot of pull. Thread maker Coats lists the zigzag stitch for bartacks in its swimwear guide.
• Design details. Because it’s decorative, it can be visible stitching you choose on purpose.

Because it’s flexible, a zigzag should stretch with the fabric. Include it in your sample stretch test. If the stitches pop around a label or stress point, flag it before you approve.

Bound, clean-finished or bonded: which edge finish should you choose?

Choose by what you want the edge to show and how hard the suit will be worn. A bound edge shows its stitching. A clean-finished edge looks smooth, with no stitching on the outside. A bonded edge has no thread at all and can feel more comfortable, but may be less durable under repeated stress. Clean-finished and bound edges are built and priced differently.

What we’ve seen: founders often send reference photos of “seamless” suits that turn out to be a clean-finished edge, not a bonded one. From the outside, the two can look the same, so ask for a photo of the inside before you assume. The difference matters for your budget, because finish choice changes your unit price. So name the finish you want, not just the look. Bali Swim makes the clean-finished edge, elastic inside with no topstitch.

Is bonded swimwear better than stitched?

Not automatically. Bonded seams can feel more comfortable, with less chafing. But a 2018 thesis on swimwear fabric found sewn seams consistently stronger and better looking than ultrasonically welded ones. Welding is another way of joining fabric without thread.

The same study found welded results too variable to recommend. Even a bonding equipment maker, Oshima, notes bonded seams may be less durable under repeated stress. Swimwear gets stretched every time it goes on and comes off, so that matters.

If you want a smooth edge, bonded vs stitched isn’t the only choice. A clean-finished stitched edge gives a smooth outside without bonding.

What thread is used for swimwear?

Bali Swim sews swimwear with polyester thread. Thread maker Coats engineers its polyester sewing threads to hold their color in chlorinated water. Swim thread has also gotten finer as fabrics got lighter.

Thread is sized in Tex, and the lower the number, the finer the thread. Load-bearing seams went from Tex 40 to Tex 35, and Tex 30 is now common. Thickness is only part of it: the seam also needs enough thread built in to stretch.

The fabric decides how much stretch a seam has to handle. Our guide to the best fabric for swimwear covers that side.

Do swimwear stitch types change your price?

Yes. Stitch and finish choices change your unit price. A clean-finished edge and a bound edge are built differently, so they’re priced differently. Ask your factory to quote both.

When you compare quotes from a swimwear manufacturer, stitching is one of the details that moves the number. Make sure every quote names the same finish, or you’re not comparing apples to apples.

At Bali Swim, Ready Design styles come with a preset stitch type. Changing it turns the Ready Design into a custom order. Shop orders start at 100 pieces in total, mixed across styles. Each style needs at least 20 pieces per color or print, with no minimum per size. Custom orders start at 250 pieces.

How do you check the seams on a swimwear sample?

Stretch every seam to the fabric’s limit and look for popped stitches or thread showing through, called grinning. Do it before you approve. Fit is approved on the sample before bulk starts.

Here’s a check any founder can do on swimwear seams:

1. Turn the suit inside out and name each stitch using the table above. Check the leg and crotch openings most closely. Lined suits have thicker seams, and these openings deserve the closest look. Compare each edge finish with your reference photos. Send your notes to the factory before you approve.
2. Stretch each seam slowly to the fabric’s limit.
3. Look for popped stitches or grinning. Either means not enough stretch is built into the seam. 
4. Check the leg and crotch openings most closely. Lined suits have thicker seams, and these openings deserve the closest look.
5. Compare each edge finish with your reference photos.
6. Send your notes to the factory before you approve.

Sampling doesn’t commit you to bulk. If something fails, fix it at the sampling stage.

Want to feel the seams yourself? The Starter Kit includes swim samples to inspect.

How do you tell your factory which stitch finish you want?

Send close-up photos of the front and back of the finish you like. Name the edge finish in words. Then agree on it at sampling, before bulk starts.

A few things make the brief clearer:

• Name the stitch. Overlock, coverstitch, flatlock or zigzag. Plain names work.
• Name the finish. Bound, clean-finished or bonded.
• Translate “seamless”. Say whether you mean a clean-finished edge or a bonded one.

For the full picture of how a line goes from idea to bulk, read how to produce your swim line.