Understanding bra cup sizing is the foundation of finding lingerie that supports, shapes, and feels comfortable. The numbers and letters on a band and cup are not arbitrary; they represent a specific mathematical relationship between your ribcage and your bust. A correct fit ensures that the weight of your breasts is distributed across the band, rather than straining the straps or digging into your sides, which is a common issue with improperly fitted bras.
The Anatomy of a Bra Size
Every bra size is a combination of two elements: the band size and the cup size. The band size, the number, provides the primary support and should sit horizontally around your torso without riding up. The cup size, the letter, indicates the difference between your bust measurement and your band measurement. This difference determines the volume of space the cups need to accommodate, which is why cup sizes are not universal across brands but relative to the band.
How the Band Size Works
The band is responsible for roughly 80% of the support, making its fit critical. To measure your band size, wrap a measuring tape snugly around your ribcage, just below your bust. If this measurement is an even number, add four inches; if it is odd, add five inches. The resulting number is your standard band size. For example, a ribcage measurement of 31 inches would suggest a 36 band, while 32 inches would suggest a 38 band.
Decoding Cup Sizing
Cup sizes are determined by the inch difference between your bust and your band. Using a standard band size of 36 as a reference, the calculations generally follow a specific scale where each cup represents roughly one inch of difference. It is essential to note that cup volume is relative; a 34D is significantly larger than a 32D, just as a 36DD is larger than a 34DD. This scaling is why comparing sizes across different band sizes can be misleading.
Common Cup Sizing Chart
The Problem with Sister Sizing
Sister sizing refers to the concept that your volume can remain the same while shifting between different band and cup combinations. For instance, a 36C contains the same volume of tissue as a 34D or a 38B. This is particularly useful when a brand fits better in a specific band size but only offers limited cup options. If a 36C fits well but the band is slightly loose, trying a 34D can create a securer fit without changing your volume needs.
