Swim Fabric 101

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Are you new to sewing swim? Or just feeling intimidated by swim fabric in general? Let us hold your hand in this comprehensive guide where we teach you the basics of Swim Fabric 101.

Swim Fabric 101

Anyone can sew swim. You don’t need any special equipment or expensive machines. Sewing swimwear is all about using the right fabrics and notions, not about using some fancy machine.

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Quality Swim Knit

One of the most important factors in creating a successful swimsuit is quality fabric. Using a high quality swim fabric means your finished suit will withstand the washing and wearing and it can be easier to sew with. A cheap flimsy fabric is almost never worth the investment because it will just lead to frustration.

Swim knit should be composed of 100% man-made fibers. Look for fabrics made with primarily polyester or nylon (80-90%) with 10-20% spandex or Lycra. These fabrics will hold their shape while being worn and won’t absorb water like a cotton spandex would.

To make a comfortable and well-fitted suit, look for swim knits that have at least 50% 4-way stretch. This is so important because of the negative ease most swimsuits have. Negative ease means the measurements of the finished swimsuit are slightly smaller than the body measurements. This ensures the suit fits snug to the body.

The lining fabric is an important part of swimsuits. It is the layer that sits directly against your skin and helps to provide added support, stability, and coverage if your main fabric is not opaque when wet.

When lining a swimsuit there are a few options:

  • For a lightweight option, look for a nylon lining fabric specifically designed for swimwear.
  • Additionally, you can also use swim knit for the lining. This does add some bulk, however, it also makes it possible to create a fully reversible swimsuit.

Right Side VS Wrong Side

If you are using a solid colored fabric, it can be hard to determine which side of the fabric is the right side.

Pull the fabric horizontally (perpendicular to the selvage) and pay attention to the “rib” lines running parallel to the selvage edge. You will be able to clearly see these lines on the right side of the fabric.

Quality Swim Elastic

Elastic needs to withstand water, chlorine, salt, and sun. Traditional polyester elastic tends to deteriorate faster under these circumstances. Instead, look for a cotton swimwear elastic. Cotton swimwear elastic is woven with rubber to give it stretch and strength. It is easy to work with and will fair well in chlorinated or salt water.

Rubber elastic is another option. It is less bulky than the aforementioned option, however, it can be a bit finicky and thus can be more challenging to work with.

Needles and Thread

While a ballpoint needle can be used on swim knit, a stretch needle is our preferred method as it helps reduce the risk of skipped stitches on fabrics with a higher amount of Lycra or spandex.

Polyester thread will not be weakened by chlorine or salt water. Quality of thread makes a big difference. Our fav is Gutermann thread.

Cutting Swim Fabric

Swim fabric is slippery and can be tricky to cut due to its shifty nature. To remedy this, cut a single layer of fabric when able to. Boo and Lu provides unfolded pattern pieces (with the option to cut on the fold if desired) on our projector pattern files to make this easier.

In the case that you do need to cut on the fold, make sure to use pattern weights. It can be anything heavy that you have in your space: cans of soup, rocks, etc. Alternatively, you can make your own DIY pattern weights using this tutorial.

Lastly, we recommend using a rotary cutter and a fresh blade for the most accurate cuts and to avoid the fabric “dragging” as you cut.   

Glue, Pins, or Clips?

Washable glue sticks are a game changer when it comes to sewing swim. Just a few dots of glue can “baste” your lining to the fabric and hold everything in place. Our favorite is this Sewline Fabric Glue Pen, but even a washable craft glue stick will do!

If you cannot get on board with the glue stick option, you can still use clips or pins. Clips are a better option but, if you prefer pins, make sure to use fine ballpoint pins to avoid damaging fabric and leaving holes. Additionally, you can also sew a basting stitch before using a regular stitch in case a mistake is made.


There’s nothing worse than pulling on a swimsuit and hearing the dreaded sound of popped stitches. To avoid this, make sure you are using a stretch stitch!

The go-to stitch that most machines have is a zigzag stitch. You can sew an entire swimsuit with nothing but a zigzag stitch, no fancy stitches or machines needed!

If you do happen to have a 4 thread serger/overlocker, this is a great option for attaching seams.

Other stitch options you can choose from, if your machine has them, are a lightening bolt stitch, triple straight stitch, twin needle, and a triple (or 3-step) zigzag stitch.


When sewing fabrics with great amounts of stretch, it can be hard to know where to start with your machine’s tension.

Sew some practice stitches on a scrap of your swim fabric to perfect the tension and stitches. You also want to be careful not to pull or stretch the fabric as you sew as this will distort the final shape of your swimsuit.

Ditch the Iron

Synthetic fabrics can melt with too much heat. If you decide to iron your swimsuit make sure your heat is not only very low, but to test on a scrap piece of fabric first. There is nothing worse than getting halfway through sewing and then ruining the project with a too hot iron.

Finger pressing can work just as well and for those stubborn seams that just won’t stay flat, you can try topstitching it down with a stretch stitch instead.

We hope this helps you have a deeper understanding of sewing swimwear. It really isn’t something to be intimidated by, once you have the right fabric, the right tools, and a little practice! If you sew up some swim we would love to see! Share with us in our Facebook group or tag us on Instagram at @booandlupatterns

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