Parts of
a bra

A bra with underwires gives the best shape and definition to a bustline. So, it’s worth the effort to try and find an underwire bra that fits you properly. Understanding underwires and how they are sized can go a long way in helping you find the right underwire bra fit.

Breasts have a diameter measurement and a projection measurement – meaning the distance the breast tissue protrudes from the chest wall. When you select a cup size such as a C or G, you are making a breast projection measurement selection. The breast diameter measurement is pre-determined by the manufacturer and built into each band size. So, changing your band size also changes your underwire diameter. Here is an example: Going from a 34C to a 36C takes you up one breast diameter measurement. Going from a 34C to a 36B keeps the same breast diameter measurement. This is why going up a band size means you also need to go down a cup size if you want to keep the same cup volume. Confused? You’re not alone.

Manufacturers use the exact same underwire size in multiple band sizes. For example, the actual underwire used in a 36B is the exact same underwire used in a 34C, 32 D1 and 30 D2. All these bras have the same breast diameter and cup volume, but different breast projection measurements in relation to the band size.

The chart below makes this a little clearer. Find your size on the chart and note the color in the box. All the other sizes in the same diagonal color stripe use the exact same underwire size. Note that Universal Cup Sizing™ is used after A through C cups. If you’re not sure what your Universal Cup Size™ is, refer to the information below the chart.

So, here’s how you can use this newfound knowledge. Say you find a bra that fits perfectly in the cups but the band is too tight. As you look at the above chart, if you follow it diagonally downward, you’ll find your A through C or UCS® with just a larger band size. The cup size went down one size as your band size went up one size. But both cups use the same underwire, so you know your cups will be the same. Conversely, if want a smaller band size but with the same cup volume, follow your size diagonally upward on the chart to get the same cup size but a smaller band size.

This chart also explains why an underwire swimsuit that is labeled as a C cup will not fit all C cup women – you really need to know what underwire size was selected to represent C cups. For the analytical types, the standard diameter deviation between underwire sizes is about 3/8". The standard underwire length deviation between sizes is about 5/8".

Underwire design is also important. An underwire is designed to have some spring. Usually made of heavy gauge wire, sheet metal or a plastic, underwires are designed to splay or spread wider once a bra is put on and fastened. It’s kind of like stretching a spring. Once a bra is removed from your body, the underwires return to their original shape. This horizontal pulling on the underwires gives additional support and containment to your breasts – especially for strapless bras.

Do you have a problem with your underwires breaking in their middle? If so, it’s because too much horizontal pulling is taking place with your underwires. Your breasts are in too small of an underwire and their weight and pressure is causing eventual breakage. Your band size could also be too small and the excessive pulling apart of the underwires is causing their eventual breakage.

Are all underwires shaped the same? For the most part, yes. But there can be some differences. Many lingerie manufacturers claim their underwire grading and shaping is their secret behind their unique fit. So, this could be a reason why you find one bra brand fitting you differently than another.

There are many different underwire lengths, as well. Demi cup, low-plunge bras and petite bra styles have shorter wires due to their design. Thus, these specific bra styles can be a great choice for a petite or short-waisted woman who has issues with underwires coming up too high in the center or under the arms. The longest underwires are used in strapless bras. Since a strapless bra does not have the benefit of straps for support, having more underwire surrounding and containing the breasts helps give the needed support to keep a strapless bra up and in place.