What you need before you start

Sewing a swimsuit is different from sewing regular clothes because swimsuit fabric stretches, doesn't fray, and needs seams that move with your body. You'll need swimsuit fabric (also called lycra, spandex, or four-way stretch), elastic made for swimwear (usually 1 to 1.5 inches wide), thread designed for stretch fabrics, and a sewing machine with a ballpoint needle or a serger if you have one. A regular needle will skip stitches on stretch fabric and break the threads.

You'll also need a swimsuit pattern. Simplicity, McCall's, and Vogue all make swimsuit patterns in multiple sizes and styles—one-piece, two-piece, high-waisted, and more. Buy the pattern in your size and read through the instruction sheet before you buy fabric, because the pattern tells you exactly how much yardage you need and what notions (elastic, thread, closures) to purchase.

A serger makes swimsuit seams stronger and more professional-looking, but it's not required. A regular sewing machine with the right needle and thread will work. You'll also want a rotary cutter and mat if you have them, though scissors work fine.

Key Takeaways

  • Swimsuit fabric requires a ballpoint needle and stretch thread on your sewing machine, or a serger, because regular needles will skip and break on lycra.
  • Cut all pattern pieces on the grain line marked on the pattern, and cut elastic to the measurement specified in your pattern instructions, not the measurement of the opening.
  • Sew seams with a stretch stitch (usually a small zigzag or a built-in stretch stitch on your machine) rather than a straight stitch, so the seam moves with the fabric.
  • Attach elastic by folding it in half lengthwise, pinning it to the inside of the opening, and sewing it down with a stretch stitch while stretching the elastic slightly as you sew.
  • Try on your swimsuit before you finish the leg and arm openings, because you may need to adjust the elastic width or tension for comfort.

Preparing your fabric and cutting the pattern

Swimsuit fabric comes folded, and it often has a shiny side and a matte side. Lay it out flat on a clean table or floor for at least an hour before you cut, so it relaxes and doesn't shift. If the fabric is wrinkled, use a low heat iron or a steamer—high heat can damage lycra.

Pin your pattern pieces to the fabric following the grain line marked on each piece. Swimsuit fabric has a direction, and cutting against the grain will make your swimsuit fit unevenly. Cut slowly with sharp scissors or a rotary cutter. Dull blades will snag the fabric and create runs. Mark all notches and construction marks with a fabric pen or tailor's chalk before you unpin the pattern.

Lay out all your cut pieces on a clean surface and double-check that you have every piece listed in the pattern instructions. It's much easier to notice a missing piece now than halfway through sewing.

Sewing the main seams with a stretch stitch

Set your sewing machine to a stretch stitch—usually a small zigzag (width 1 to 1.5, length 2 to 2.5) or a built-in stretch stitch if your machine has one. Thread your machine with stretch thread in the bobbin and regular thread on top, or use stretch thread on both if you prefer. A straight stitch will snap when you put on the swimsuit because the fabric stretches and the thread doesn't.

Pin the first seam (usually the shoulder seams on a one-piece) with the right sides of the fabric facing each other. Sew slowly and steadily, keeping the seam allowance even. Most swimsuit patterns call for a half-inch seam allowance. If you have a serger, use it instead—a serger seam is stronger and more flexible than a machine-sewn seam on stretch fabric.

After you sew each seam, try it on or hold it up to your body to check the fit. Swimsuit fabric doesn't have much give once it's sewn, so if a seam is too tight, you'll need to pick it out and resew it. Sewing the side seams last gives you the most room to adjust if needed.

Attaching elastic to openings

Elastic is what holds a swimsuit in place. Your pattern will tell you the length to cut the elastic—this is usually shorter than the opening itself, because you'll stretch the elastic as you sew it on. For example, if the leg opening measures 20 inches, the pattern might call for 18 inches of elastic. Always follow the pattern measurement, not the opening measurement.

Fold the elastic in half lengthwise (so it's now half as wide) and pin it to the inside of the opening with the raw edges of the elastic aligned with the raw edge of the fabric. The fold of the elastic should face inward. Divide both the elastic and the opening into quarters and mark them with pins so the elastic stretches evenly.

Sew the elastic down using your stretch stitch, stretching the elastic gently as you sew so it lies flat and doesn't bunch. Start at one marked quarter and sew to the next, stretching the elastic between pins. This takes practice—if the elastic bunches, you stretched too much; if it wrinkles, you didn't stretch enough. After you sew, fold the elastic to the inside and sew it down again with a stretch stitch to find it permanently and create a finished edge.

Trying on and adjusting before finishing

Before you attach the elastic to the leg and arm openings, try on your swimsuit. This is your final note to adjust the fit without picking out seams. Check that the shoulder seams sit where you want them, that the neckline isn't too tight, and that the side seams are comfortable. If something feels wrong, mark it with a pin and take it off to adjust.

If the swimsuit is too loose in the chest or hips, you can take in the side seams by picking out a few inches and resewing with a narrower seam allowance. If it's too tight, you may not be able to let it out much—swimsuit fabric doesn't have much extra room. This is why trying on before you finish the openings matters.

Finishing the leg and arm openings

Once you're happy with the fit, attach elastic to the remaining openings using the same method: fold, pin, divide into quarters, sew with a stretch stitch while stretching gently, then fold and sew again. Work on one opening at a time so you don't get confused about which elastic goes where.

After all the elastic is attached and sewn down, trim any loose threads with small scissors. Check all the seams to make sure the elastic is fully enclosed and no raw edges are showing. If you see a gap where the elastic didn't get caught in the seam, you can hand-stitch it closed with a needle and thread, or pick out that section and resew it.

Common mistakes and how to avoid them

The most common mistake is using a regular needle and regular thread, which causes skipped stitches and broken threads. Buy a ballpoint needle and stretch thread before you start—they cost a few dollars and make the difference between a swimsuit that holds together and one that falls apart.

The second mistake is sewing elastic on too tightly or too loosely. If you're not sure, sew it on loosely the first time, try on the swimsuit, and resew if needed. It's easier to tighten elastic than to loosen it.

The third mistake is not reading the pattern instructions all the way through before you start cutting. Patterns sometimes call for fabric to be folded a certain way or for pieces to be cut on the bias. Cutting wrong means wasting fabric and starting over.

Frequently Asked Questions

Can I use regular fabric instead of swimsuit fabric?

No. Regular cotton or polyester doesn't stretch, so the swimsuit will be stiff and uncomfortable. It will also absorb water and take a long time to dry. Swimsuit fabric is designed to move with your body and shed water quickly.

What if I don't have a serger?

A serger makes seams stronger, but you can sew a perfectly good swimsuit with a regular sewing machine if you use a ballpoint needle, stretch thread, and a stretch stitch. The seams won't look quite as professional, but they'll hold.

How do I know what size pattern to buy?

Measure your bust, waist, and hip with a soft measuring tape and compare to the size chart on the pattern envelope. Swimsuit patterns are usually sized the same as regular clothing patterns, but always check the chart because some brands run small or large.

Can I add padding or a built-in bra to my swimsuit?

Yes, but you need to buy swimsuit padding or a bra cup insert designed for this purpose. Regular bra padding will absorb water. Sew the padding to the inside of the cup before you sew the front and back pieces together, following the padding manufacturer's instructions.

What do I do if the elastic keeps rolling or twisting?

Elastic rolls when it's not sewn down securely on both edges. Make sure you're sewing it twice—once when you first attach it, and again after you fold it to the inside. If it still rolls, you can hand-stitch it down with a needle and thread along the edges.