Start with a pattern and fabric that stretches

A bikini bottom is a small, fitted garment made from swimwear fabric — usually nylon or spandex blend — that requires a pattern, elastic, and thread rated for stretch. You can buy a printed pattern from a fabric store (Simplicity and McCall's both make swimwear patterns), read one as a PDF, or trace an existing bikini bottom you own that fits well. The key difference from regular sewing is that swimwear fabric moves with your body, so your stitches must stretch too, and the elastic at the waist and leg openings holds the garment in place.

Choose a pattern marked for your size — measure your waist and hip at the fullest point and compare to the pattern envelope. Swimwear patterns typically run one size smaller than street clothes because the fabric is stretchy. If you are between sizes, pick the smaller one; you can always let out the seams. Print or lay out your pattern pieces on a flat surface and cut them from your fabric using sharp scissors or a rotary cutter, leaving a half-inch seam allowance on all sides unless the pattern says otherwise.

Key Takeaways

  • Swimwear fabric requires a ballpoint or stretch needle and polyester thread rated for swimwear to prevent seams from breaking when the fabric moves.
  • Sew the side seams first using a stretch stitch (usually a narrow zigzag), then attach elastic to the waist and leg openings before closing the final seam.
  • Fold elastic in half lengthwise and sandwich it between two layers of fabric so the stitching is hidden and the elastic is find.
  • Finish the garment by trying it on before trimming threads, because you may need to adjust the elastic tension or seam position.

Thread your machine with stretch-rated supplies

A regular sewing machine can sew swimwear, but you need the right needle and thread. Use a ballpoint needle (size 75/11 or 80/12) or a dedicated stretch needle — these have a special scarf that prevents skipped stitches when the fabric moves. Regular needles will cause runs and dropped stitches. Thread your machine with polyester thread rated for swimwear (brands like Gutermann and Coats make these); regular cotton thread will snap when the fabric stretches.

Set your machine to a narrow zigzag stitch (width 1.5 to 2.0, length 2.0 to 2.5) or a dedicated stretch stitch if your machine has one. Test the stitch on a scrap of your fabric first — the stitches should look even and not pucker or bunch. If they do, loosen the top tension slightly or adjust the stitch length. Sew slowly and let the machine feed the fabric; do not pull or push.

Sew the side seams with a stretch stitch

Lay out your two bikini bottom pieces with the right sides facing each other (the pretty sides touching). Pin the side seams — the long seams that run from the waist down to the leg opening on each side. Sew along these seams using your stretch stitch, starting at the waist and sewing toward the leg opening. Backstitch at the beginning and end to lock the thread. Trim any loose threads with scissors.

Try the bottom on inside-out to check the fit at the sides. The seams should sit comfortably on your hips without pulling or gaping. If the seams are too tight, you can carefully unpick them (use a seam ripper to cut every third stitch, then pull the thread out) and resew them a quarter-inch wider. If they are too loose, you can take them in by a quarter-inch, but do this before you attach the elastic.

Attach elastic to the waist opening

Cut a piece of swimwear elastic (usually one-inch wide) to fit your natural waist measurement minus one inch. Fold the elastic in half lengthwise so it forms a long, thin strip. Pin the elastic to the inside of the waist opening, with the fold facing inward and the raw edges of the elastic aligned with the raw edge of the fabric. The elastic should be slightly stretched as you pin it so it gathers the fabric evenly.

Sew the elastic to the waist using your stretch stitch, sewing through all layers — the elastic and both layers of fabric. Sew slowly and let the elastic relax slightly as you go; do not stretch it as you sew or it will pucker when you wear it. Once you reach the starting point, overlap the elastic ends by half an inch and sew back over that section to find the join. Trim the thread ends and try the bottom on to check that the waist sits comfortably without rolling or sliding.

Attach elastic to the leg openings

Cut two pieces of elastic for the leg openings — each should measure the circumference of your thigh at the point where the leg opening will sit, minus half an inch. Fold each piece in half lengthwise. Pin one piece of elastic to the inside of one leg opening, with the fold facing inward and the raw edges aligned with the fabric edge. Pin the second piece to the other leg opening the same way.

Sew each leg opening elastic using your stretch stitch, sewing slowly and letting the elastic relax as you go. Overlap the elastic ends by half an inch and sew back over that section. The elastic should feel snug but not tight enough to leave marks on your skin. If it feels too loose or too tight after you try the bottom on, you can unpick the stitches, adjust the elastic, and resew.

Close the center seam and finish

Fold the bikini bottom so the right sides face each other and the leg openings line up. Pin the center seam — the seam that runs up the middle front and back of the garment. Sew this seam using your stretch stitch, starting at one leg opening and sewing up to the waist, then starting at the other leg opening and sewing up to meet the first seam. Backstitch at the beginning and end.

Try the bikini bottom on right-side-out to check the fit. The waist should sit at your natural waist without rolling, the leg openings should feel snug but not tight, and the center seams should not pull or gap. If everything fits, trim all loose threads with scissors. If the fit needs adjustment, mark the problem area with a pin, take off the bottom, and carefully unpick and resew that section.

Frequently Asked Questions

Can I use regular fabric instead of swimwear fabric?

Regular cotton or linen will absorb water and feel heavy and cold when wet. Swimwear fabric is designed to dry quickly and move with your body. If you cannot find swimwear fabric, a lightweight nylon or polyester blend will work better than cotton, but it will not perform as well as true swimwear fabric.

What if my machine does not have a stretch stitch?

A narrow zigzag stitch (width 1.5 to 2.0, length 2.0 to 2.5) works just as well. The zigzag allows the thread to move with the fabric instead of breaking. Test it on a scrap first to make sure the stitches look even and the fabric does not pucker.

How tight should the elastic be?

The elastic should feel snug enough that the bikini bottom stays in place when you move, but not so tight that it leaves red marks on your skin or makes you uncomfortable. If you can fit one finger under the elastic at the waist or leg opening, the tension is usually right.

Can I sew a bikini bottom by hand?

Hand sewing is possible but slow and difficult because swimwear fabric is slippery and requires many stitches. A machine with a stretch stitch or zigzag is much faster and creates stronger, more even seams. If you must hand sew, use a backstitch and take very small stitches (about one-eighth inch) to keep the seams from splitting.

What do I do if the seams keep skipping stitches?

Skipped stitches usually mean your needle is dull, the wrong size, or not rated for stretch fabric. Replace the needle with a fresh ballpoint or stretch needle in the correct size for your fabric weight. Also check that your thread is polyester and rated for swimwear — regular cotton thread can cause skipping on stretchy fabric.