How to Sew Swimwear with Bra Cups: A Step-by-Step Guide

By Les Tissées – Curated Fashion Fabrics | Published: 2026-10-19

Category: How-to Guides

Learn how to sew swimwear with bra cups, including how to insert triangle bra inserts for bikinis and sports bras. Step-by-step instructions for a professional finish.

Quick answer

To sew swimwear with bra cups, choose lightweight foam inserts like triangle bra inserts, sew a secure channel or pocket into your lining, and insert them after the main seams are done.

Why foam inserts beat sewing cups into the seam

Sewing a bra cup directly into a seam can create bulk and make the cup sit oddly. Instead, sew a lining pocket or channel that holds the cup in place. This keeps the outer fabric smooth and lets you remove the cups for washing.

Foam inserts are ideal for swimwear because they dry quickly and resist chlorine. A 2.4 mm thick insert provides light shaping without adding heavy bulk, which is especially useful for bikinis and sporty tops.

Choosing the right cup for your project

Triangle bra inserts work well for triangle bikinis and soft-cup bras. They are lightweight and quick-drying, making them a practical choice for swimwear. For a fuller-coverage top, you might want a larger cup, but the same insertion method applies.

Consider the color of the insert if your swimwear fabric is light or white. Nude inserts blend better under pale fabrics, while black inserts work under dark colors. Always test the insert against your fabric before sewing.

Step-by-step: Sew swimwear with bra cups

Prepare your pattern and fabric: Cut your swimwear pieces from both outer fabric and lining. Mark the placement of the bra cups on the lining side, using the pattern's notches or your own measurement. Check that the cup fits within the seam allowance. Always test the cup placement on a muslin or scrap fabric before cutting your final fabric.
Sew the cup pocket: On the lining piece, sew a horizontal line across the bottom of the cup placement, then sew two vertical lines to create a pocket. Leave the top open so you can insert the cup later. Use a stretch stitch or a narrow zigzag to keep the fabric flexible. Make the pocket slightly larger than the cup so it doesn't pucker.
Insert the bra cup: Slide the foam insert into the pocket through the top opening. Smooth it flat and adjust so it sits evenly. If the cup has a specific shape, make sure the point faces the correct direction (usually toward the center of the body). Check that the cup lies flat and doesn't wrinkle the lining.
Close the pocket opening: Hand-stitch the top opening closed with a ladder stitch, or edge-stitch it on the machine. If you prefer a removable cup, skip this step and leave the opening unstitched, but make sure the opening is small enough to keep the cup secure. For a removable cup, use a small opening with a secure closure like a snap or hook-and-loop tape.
Assemble the swimwear: Sew the lining to the outer fabric along the top edge and side seams, leaving the bottom open. Turn the garment right side out, then sew the bottom hem. The cup pocket should be between the outer fabric and lining, so the cup stays hidden. Press the seams carefully to avoid flattening the foam.

Product facts

Triangle Bra Inserts | Swimsuit, Bikini & Sports Bra Cups | Made in Quebec: These triangle bra inserts are made of lightweight foam, are chlorine resistant, and quick drying. They are ideal for swimwear because they won't absorb water or break down from pool chemicals.
Triangle Bra Inserts | Swimsuit, Bikini & Sports Bra Cups | Made in Quebec