There are a lot of similarities between a pair of underwear and a bikini bottom, so why not use a sewing pattern for one to make the other? If you’ve got a swimsuit bottom that you like there’s no reason that you can’t use it to make a stack of underwear as well! I’ve put together a few things to keep in mind when doing so – pattern selection, fabric selection, and three different ways to finish the edges of your underwear.

Fabric Selection: Swimsuit patterns are designed to be sewn with swimsuit fabrics which have some key characteristics: 4-way stretch, great recovery, and some heft. You clearly don’t want to sew your underwear out of swimsuit fabric, but when you pick a jersey, you should keep these characteristics in mind. You don’t need a 4-way stretch jersey, but you do want to make sure that your jersey has good stretch to it. The jersey should also have good recovery so it doesn’t droop off your body with wear. Finally, I suggest using a medium weight jersey so that it is strong enough to hold the shape of the pattern – use your lightweight jerseys on patterns that were specifically designed to be underwear.

Pattern Selection: The fit that you like in a swimsuit pattern may be different than the fit you like in an underwear pattern, so pause for a minute to consider whether your swimsuit pattern would actually translate well to underwear. The Nautilus Swimsuit bikini bottom happens to be pretty much exactly what I look for in underwear (one of the perks of being the designer!) because I like full rear coverage on both – but I know plenty of women that like full cut swimsuits like skimpier undies. This isn’t earth shattering advice, but I know how tempting it is to think “oh I have this type of pattern already” and jump right in to sewing it without stopping to consider whether you will like the end result.

How to add a crotch lining: Underwear differs from swimsuits in that it has a crotch lining, but adding one into your pattern is really quite simple. Using the front piece of your swimsuit pattern, cut out the bottom several inches for a second time. The exact length that you will want to use is dependent upon where the seam between front and back hits on your body and how long you like your linings to be. A good rule of thumb is to cut the lining before the front pattern starts to widen out.

Sew the front of the underwear to the back at the crotch in a stack of front right side up, back right side down, and lining right side down.

Flip the lining toward the front so the seam is encased. Fold the raw end of the lining under and stitch it down.

Finishing the edges – Option 1: You will probably want to finish the edges of your underwear differently than you do on a swimsuit. A quick and easy way to do so is to use strips of 1″ or 1 1/2″ jersey. Fold the strips in half, wrong sides together, and sew them around the openings of your undies, stretching the strips a bit as you sew.

Then flip the raw edges to the inside and topstitch along the underwear to hold the bands in place. When you sew the band to the underwear openings, you can do so by using a straight stretch stitch or narrow zig-zag with a 1/4″ seam allowance or use a wide zig-zag. If your swimsuit pattern calls for you to insert stretched elastic at any point, make sure you do so when sewing the underwear to get the same fit. In this sample, I inserted a layer of elastic between the folded binding on the cheek and used a wide zig-zag to sew over it for the portion of the Nautilus Swimsuit pattern that calls for stretched elastic.

Finishing the edges – Option 2: There are many ways to sew bindings on the edge of your underwear. Another way is to sew the binding to the underwear with the binding open, right sides together (again including elastic only where it is used to gather).

Then wrap the binding toward the back and topstitch it down from the right side. If your binding is too wide on the inside, you can trim it down close to the seam allowance. This method is similar to a Hong Kong finish and distributes the bulk a little differently than the previous finish.

Finishing the edges – Option 3: A third option  is to use a picot edge or other decorative lingerie elastic to finish the edges of your suit. This provides the cleanest inside finish but does require buying specialty elastic.

Sew the picot elastic right sides together to the edge of the underwear. The edge of the fabric should be within the elastic. If sewing a seam this narrow is tricky, you can sew the a 1/4″ seam and then trim the fabric down.

This is what the first seam looks like from the other side. The stitch line should run just inside the picot.

Then fold your elastic to the inside and topstitch to hold it in place. Just the cute little picot should be visible from the right side.