Tomima's Blog – Lingerie & Underwear Expert

Have you ever bought an underwire bra in your size and felt like the underwires didn’t fit right? This posting is to help you understand bra underwires and how they are designed, selected and ultimately sewn into your bras.Let’s begin with some basic geometry. Breasts, for the most part, are shaped like orbs. They have a diameter and a projection. When selecting your cup size (e.g. B, DD, G, etc.) you’re basically choosing a size that addresses your breast projection — how far out from your body your breasts project.

Your breast diameter measurement, on the other hand, is already predetermined by the manufacturer who makes your bra. Your band size (32, 38, 42 etc) has a pre-determined breast diameter built into it in the form of an underwire. That’s right, manufacturers determine your breast diameter whether you like it or not, when they decide on the underwire that will be sewn into your bra. And all lingerie manufacturers pretty much follow the same sizing conventions with regard to diameters. As a side note, augmented breasts tend to have a wider diameter than their natural counterparts. This is why many augmented women tend to wear larger cup sizes than they thought they would wear – it’s to get the correct fit with a wider underwire.

So, now that you know this, you can understand why changing your band size will also change your breast fit. And here’s another industry secret. Manufacturers use the same underwire in different bra sizes. This is done to reduce the number of different underwires they need to work with., but it also makes sense. Remember, your cup size is determining your projection. A woman with a larger band size, but with the same projection should also have the same diameter – in theory. Here’s an example to explain how all of this works. Suppose you are a 36C. The industry has quantified over the years that your projection is the same measurement as a woman who is a 32DD, 34D, 38B and a 40A. So, most of the time, these bra sizes will all have the same underwire sewn into them. This makes sense when you consider what you do with weight fluctuation. If you are in a 36C and the cups fit, but the band is too tight, the correct size change would be to go up a band size to a 38 and down a cup size to a B to maintain the same cup volume. After all, your girth changed; not your breast diameter.

I put this chart together to guide you in all of this. So, the next time you feel you need to change your bra size, stop and think about what needs to change. If it’s just your band, consult with this chart to find the next size you should consider moving to.

Underwire shapes can and do vary among brands and manufacturers. In fact, many feel their underwire grading and shaping choices are the trade secret to their fit. This is why you’ll find slight underwire variations among manufacturers (however they will not tell you or us what they are). This explains why one manufacturer may fit you better than another.

Underwires also have different lengths – their curve will follow the same size circle, but, like a pie chart, the diameter length can vary. For example, demi cup bras, balconette bras, bras labeled "petite" and low-plunge bra styles generally have shorter wires. If you are short in stature, this is good to know, because a shorter wire will have a less chance of poking you under your arm since these wires do not come up as high. Strapless bras tend to have the longest underwires, and for good reason. Without strap support, a strapless bra requires more wire to circle around the breast in order to give proper support and keep the bra up.

Now that you understand underwire sizing, you can understand why underwire swimsuits with just a cup size like a "C" don’t fit all C cup women. And, for you analytical types who are probably wondering, when underwires are graded to create their different sizes, the diameter increase is approximately 3/8" and the underwire length increase is approximately 5/8". Underwires are also engineered to have some spring. Once a bra is fastened, horizontal pressure is exerted on the underwires so that they splay or spread. Then, when a bra is unhooked, the underwires return to their original shape. This springing or splay gives additional support to your breasts by exerting inward pressure on them – like a gentle squeeze. If you are in the wrong size bra such that your breasts are wider than the splayed diameter of the underwire, over time the pressure and weight of your breasts can cause an underwire to break in half. If this happens to your frequently, you are probably in a bra with too small of a band size, or one where the cups are too small.

I appreciate that learning all of this may be a little overwhelming. But, you should also know that manufacturers work very hard to get you the best fit possible and try and make your size selection easy. However, it’s always nice to know the logic behind what they do.

Learn more about underwires and the perfect fit in the HerRoom fitting guide.