Anatomy of a Bra: Underwires

Jacqueline Wells

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 perfect underwire bra fit.

Breasts have a diameter measurement and a projection measurement – distance the breast tissue protrudes from your chest wall. When you select a cup size like 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, 32D and 30E. All of these bras have the same breast diameter; just different breast projection measurements in relation to the band size – but all have the same cup volume.

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 the bra is removed from your body, the underwires return to their original shape. This horizontal pulling on the underwires gives you additional support and containment to your breasts – especially so 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 an underwire and their weight and pressure are causing eventual breakage. Or your band size is too small and the excessive pulling apart of the underwires causes 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 to be their secret behind their unique fit. So, this could be a reason why you find one bra brand fitting your 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 their 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 breast helps give the needed support to keep a strapless bra up and in place.