Adding cups to a swimsuit pattern is actually an easy modification that can make your suit instantly more supportive and comfortable. It’s pretty easy to slip swimsuit or bra cups into a suit or with a bit more work you can even add a cup with an underwire!
Choosing cups: There are many kinds of different bra cups out the biggest consideration you need to make is whether it’s the type of cup that is comfortable to you. You can buy softer cups that don’t give a lot of support but do provide a bit of modesty. Foam cups are a great choice for both support and modesty. If your foam cups aren’t specifically labeled for swimsuits, make sure that they don’t retain a lot of water like a sponge when they are wet! I find one of the best ways to source cups is to go to a thrift store and buy a very lightly used swimsuit and cut out the existing cups.
Sizing cups: Choosing a swimsuit cup size is much less exacting than picking a bra size so generally you will find them in a multiple cup size range (e.g. A/B or C/D). You can generally go by your bra size for size selection although my personal experience has shown that the cup will stay in place better and provide more comfort and coverage in your suit if you choose a cup that is a bit too big instead of a bit too small. For foam cups, you can easily trim them down to fit if you buy ones that are too big, although you can’t go down more than a cup size or two because of the shape of the underlying curve of the cup.
Inserting cups: Inserting bra cups into a swimsuit is as simple as sliding them in between the fashion fabric and lining. Make sure you do this during construction before you have enclosed all the side of the cup! If you forget or find that your suit needs cups after you are done, you can cut a slit in the lining along the side and slip in a cup after the fact. Depending on your cup, bust, suit, use, and fit, you may find that the cups stay in place without a problem or you may find that they slide around a bit. If they tend to slide out of place, try on the suit and pin the cups in place. Make a couple of tacks through the lining into the cup to keep them permanently situated in the correct location.
Lightweight alternative: If you don’t need much extra support and are just looking for a bit of modesty for when it gets chilly, you don’t need to insert a cup. Instead, cut out several layers of lining fabric and sew them as one for the cup lining. Multiple layers of fabric is typically preferred by smaller and/or perkier busted folks as it doesn’t do much for support.
Inserting an Underwire Bra Cup: If you have a well fitting bra (foam cup, with or without underwire) that you are will to sacrifice, you can easily insert it into your swimsuit as well. Cut your bra in half through the center and cut off the straps. Cut an X in the cups of your swimsuit lining (the size of the X will vary with your bust size), push the bra cups up into the X, and pin them in place.
Sew the lining to the cups, being careful not to hit your underwire and use the lining piece in your swimsuit construction. You can machine sew through the cup itself or hand sew around the edge of cup.
To prevent visible lines from the front, you may want to cut away the excess lining fabric. If you want a pretty interior finish, you can cut a second piece of lining and use it on the inside of the sewn-in cups.
*Thanks so much to Elaine for the idea and photos of inserting a bra into her swimsuit. Read her mini-tutorial on Curvy Sewing Collective for more.*