What You Need Before You Start

Sewing bikini bottoms is simpler than sewing most garments because the pieces are small and the seams are straightforward. You will need a sewing machine (any basic model works), thread in a color that matches your fabric, elastic for the waistband and leg openings, and fabric—usually a stretch knit like lycra, nylon, or spandex blend. Most bikini bottoms use between 0.5 and 1 yard of fabric depending on the style and your size.

You will also need a pattern. You can buy printed patterns from Simplicity, McCall's, or Butterick (search "bikini bottoms" on their websites), read PDF patterns from sites like Etsy or Craftsy, or trace an existing pair of bottoms that fit you well. A pattern is essential because it gives you the exact pieces and measurements for your body.

Before you buy fabric, check whether your sewing machine has a ballpoint needle and stretch thread or woolly nylon thread. Stretch fabrics need these supplies to prevent skipped stitches and broken seams. If your machine does not have them, a ballpoint needle costs a few dollars at any fabric store.

Key Takeaways

  • Bikini bottoms require a stretch-fabric pattern, a ballpoint needle, and elastic—standard sewing machines can handle them if you use the right needle and thread.
  • The basic construction is four seams: two side seams, one center front, and one center back, followed by elastic casings at the waist and leg openings.
  • Stretch fabrics need a ballpoint needle and a stretch stitch (or a narrow zigzag) to prevent seams from breaking when the fabric moves.
  • Elastic width and type vary by style—waistband elastic is usually 1 to 1.5 inches wide, while leg elastic is 0.5 to 0.75 inches.

Cutting Your Pattern Pieces Correctly

Lay your pattern pieces on the fabric following the grain line marked on the pattern. Stretch knits have a direction—the fabric stretches more in one direction than the other—and the pattern is designed to use that stretch in a specific way. If you ignore the grain line, your finished bottoms will pull or sag in the wrong places.

Pin the pattern to the fabric and cut slowly with sharp scissors or a rotary cutter. Stretch fabrics fray less than woven fabrics, but clean edges still matter for seaming. Mark the notches (small cuts on the pattern edges) with a fabric pen or tailor's chalk—these notches tell you where seams meet and where to attach elastic.

Most patterns include a front panel, a back panel, and sometimes a gusset (a small reinforcement piece for the center seam). Cut all pieces and lay them out in order before you start sewing. This prevents mistakes and saves time.

Sewing the Side Seams and Center Seams

Start by sewing the two side seams. Place the front and back panels with the right sides together (the pretty sides facing each other) and pin along the side edges. Use a ballpoint needle and set your machine to a stretch stitch—this is usually marked with a lightning bolt or wave symbol on your machine dial. If your machine does not have a stretch stitch, use a narrow zigzag stitch (width 1 to 1.5, length 2 to 2.5) instead. Sew slowly and let the fabric feed naturally without pulling.

After both side seams are sewn, sew the center front seam and center back seam the same way. If your pattern includes a gusset, insert it at the center front seam by sandwiching it between the two fabric layers before sewing. Press all seams flat with a warm iron—do not use high heat on stretch fabrics, as it can melt them.

Try the bottoms on inside-out at this point. The leg openings and waist should sit where you want them. If they are too loose or too tight, you can adjust the seam allowance slightly on the next pair, but for now, move forward with elastic.

Attaching Waistband Elastic

Cut a piece of elastic to fit your waist comfortably—it should stretch slightly when you put it on but not dig in. Most people use elastic that is 1 to 1.5 inches wide. Measure your waist, subtract 1 to 2 inches (the elastic will stretch), and cut the elastic to that length. Overlap the ends by 0.5 inches and sew them together with a zigzag stitch to form a loop.

Turn the bottoms right-side-out. Divide both the waistband opening and the elastic loop into four equal sections and mark them with pins. Match the pins and pin the elastic to the inside of the waistband, with the elastic sitting 0.25 inches below the raw edge. Sew the elastic to the fabric using a zigzag stitch, stretching the elastic as you sew so it lies flat. This creates a casing that holds the elastic in place.

Fold the raw edge of the waistband down over the elastic and sew it flat with another zigzag stitch. This finishes the waistband and prevents the elastic from rolling.

Attaching Leg Elastic

Leg elastic is narrower than waistband elastic—usually 0.5 to 0.75 inches wide. Cut two pieces, one for each leg opening, using the same method as the waistband: measure the leg opening, subtract 0.5 to 1 inch, and overlap the ends by 0.5 inches before sewing them into loops.

Pin the leg elastic to the inside of each leg opening, dividing both the opening and the elastic into four sections and matching the pins. Sew the elastic with a zigzag stitch, stretching as you go. Fold the raw edge down and sew it flat the same way you did the waistband.

Try the bottoms on right-side-out. The elastic should sit snugly without leaving marks on your skin. If the leg elastic is too tight, you can carefully cut it out and replace it with a slightly longer piece—this is a common adjustment on the first pair.

Finishing Touches and Troubleshooting

Trim any loose threads and press the finished bottoms with a warm iron on the lowest setting. Wash them in cool water with mild detergent before wearing to remove any marking pen or chalk. Stretch fabrics last longer if you wash them inside-out and hang them to dry rather than using a dryer.

If your seams are puckering (bunching up), you likely used a regular needle instead of a ballpoint needle, or your stitch length was too short. Ballpoint needles slide between the fibers of stretch fabric instead of piercing them, which prevents this problem. If you skipped stitches, the same issue applies—switch to a ballpoint needle and a stretch stitch.

If the waistband or leg openings are too loose, the elastic was too long or not stretched enough while sewing. On your next pair, cut the elastic 1 to 1.5 inches shorter than the opening measurement instead of 0.5 to 1 inch shorter. If they are too tight, add 1 inch to the elastic length.

Frequently Asked Questions

Can I use a regular sewing machine for bikini bottoms?

Yes, but you must use a ballpoint needle and a stretch stitch or narrow zigzag. Regular needles pierce stretch fabric and cause seams to break when the fabric moves. A ballpoint needle costs a few dollars and is the only special supply you absolutely need.

What if I do not have a pattern?

Trace an existing pair of bikini bottoms that fit you well. Lay them flat on paper, trace around the edges, and add 0.5 inches all around for seam allowance. This gives you a custom pattern that will fit your body. Mark where the elastic goes and where the seams meet.

How much elastic do I need?

Measure your waist and each leg opening. For waistband elastic, cut a length that is 1 to 2 inches shorter than your waist measurement. For leg elastic, cut each piece 0.5 to 1 inch shorter than the leg opening. These numbers account for the stretch of the elastic when sewn.

What thread should I use?

Regular polyester thread works fine for bikini bottoms. If you have access to stretch thread or woolly nylon thread, either one prevents seams from breaking when the fabric stretches, but they are not required if you use a stretch stitch or narrow zigzag.

Why are my seams puckering?

Puckering happens when a regular needle pierces the fibers of stretch fabric instead of sliding between them. Switch to a ballpoint needle and use a stretch stitch or narrow zigzag. Also check that your stitch length is not shorter than 2—very short stitches bunch up stretch fabric.