Your breast is nothing more than a mound of flesh attached to your body. That mound has a diameter at its base and reaches a height that is unique for every woman. Underwires are meant to encircle that base and help to support it within the cup. The important thing to remember is that the diameter of the breast determines the underwire size, and that unique diameter, can be used to fit more than one bra size.
To explain further, your bra size is made up from a combination of band size and cup size. As the breast grows larger in diameter, it takes more breast mass to make up the same height, so that a 36B breast is much smaller than a 46B breast, yet each of these, being B cups have only 2″ difference between the high bust and the full bust. Predictably, a 36B bra takes a size 36 underwire, while the 46B bra (which is still a B cup) will take a size 46 underwire, which has a diameter 2″ bigger! This explains why women who buy swimwear cups by cup size “I wear a B cup” may find that the B cups do not really fit at all, especially if they are at the lower or upper end of the size range.
Although it may sound a bit confusing, the diameter system for underwires actually makes life easier for a bra-maker, as well as those salvaging wires from ready-to-wear. Since each diameter of wire is unique to a bra size, it also follows that wires may be interchangeable between some sizes. For example, a 36B takes a number 36 underwire, but so do a 32D, a 34C and a 38A! Use the chart below to determine a particular underwire size. The cell where your band size meets your cup size is your underwire size. Notice that as the band size increases, the cup size that will accommodate the wire decreases.
The chart goes up to 66, but to my knowledge this underwire size does not exist. Instead I have used the theoretical diameter of this size. It’s been my finding that women with larger breasts (larger than 58) are not comfortable wearing wires, since the mass of the breast would extend so far under their arms.
You will also notice that the sizes move in a diagonal direction, as the band size increases, the cup size decreases. So the same number wire, for example, wire size 40 will fit not only 40B, but also 38C, 36D, 34DD, and 32F. All these sizes have the same diameter around the base of the breast mound. However this does not mean that the volume of the breasts is the same. The thrust (depth) is different, even though the diameter may be the same.
You will notice, too, that all the wires are numbered for their size in a B cup, so that is an easy way to remember them. Each increase in cup size causes the wire to go up by 2, so a 36 wire is only 1 size up from a 34. The actual size difference is approximately 8.5 mm in diameter.
The 30-second Wire Test
If you really want the naked truth about breast diameter, try this quick tip. Hold up an underwire so it encircles the bare breast with the coloured tip of the wire at the front. If the underwire intrudes into the breast mound, it is too small. You can see below that the wire is too small and sits on the breast mound. This would dig into the breast and be quite painful.
If the wire diameter goes beyond the diameter of the breast, as shown below, it is too large. Underwires increase in diameter by increments of approximately 1/3″ per size so you can judge how many sizes you need to move up or down.
Once you have the wire that fits, when you look at bras in the store, make sure the wire diameter matches the diameter of “your” wire, or very close to it. It is easy to check by turning the bra inside out and laying your wire up to it. After a few times, you’ll have memorized the size and you won’t have to carry a wire around in your purse!
In order to check underwires that may already be in your possession, simply lay them on top of the wire that is correct for you. Most women assume that if they have underwires with the same colour tips, they are getting the same size, but colours run totally differently between manufacturers. At one time, I had 4 different wire sizes, all with green tips!
The curve that most closely matches the curve of your wire indicates the size. Do not worry so much about the arm length; your wires may have been taken from a demi-cup bra or bra style with slightly less than full coverage. What is critical, however, is that the diameters of the wires in question match up as closely as possible.