In some areas of life, like baking, how much volume is required to fill a cup is fairly straightforward – 250cc. Unfortunately when it comes to breast implants, it is not quite as straightforward. Despite about 1/2 the population wearing a bra on a daily basis, very few people actually know what the measurement means and how it is determined. Bra size consists of two measurements – the cup and the band. Band size is determined by the measurement around just the ribcage, usually at the level of the inframammary crease (where the band usually sits). This is the 32, 34, etc… in the bra size. Cup size is then determined by measuring around the entire chest at the point where the breasts project the most and subtracting the band size from this number. An A cup indicates a difference of one inch between these measurements, a B cup means a difference of 2 inches and so on. Putting these two measurements together is how someone ends up with a bra size of 32B or 42DD. Because cup size is relative to the band size, a person with a small band will require less volume added (a smaller implant) to go up in cup size than someone with a larger band. This magic number will also change with the starting cup size as well: to increase from an A cup to a B cup will require a smaller implant than going from a C to a D. This is because breasts are not rigid structures, and the added volume gets diluted throughout the soft tissues as the amount of soft tissue increases.
Finally, breast implants also come in different projections, which means that for any given breast a 250cc low profile implant will effectively increase the cup size less than a high projection implant, even though the exact same amount of volume has been added. This is why there is no absolute formula that allows me to tell my patients that, for example, every 127cc of silicone added will result in a one-cup increase in your bra size.
To further complicate matters is the fact that cup size differs wildly between manufacturers. One woman may be a double D at Victoria’s secret, and a C at La Senza. For all these reasons, I suggest that a better way to choose your breast implants is to try on sizers and simply see what you like best, ignoring the specific cup size. For more information, see my blog post on how to choose breast implants.