Relative Bra Size ~ A (not so) fundamental fact.

It is most interesting to watch the reactions of people when I explain that I have spent the last  six years researching breast mound sizing.  Only a few have had the curiosity to ask what there was to study for that period of time and those few were quickly sorry because  I can discuss the topic for longer than most care to listen.   As I was reminded of last week, however, there is still much to talk about.

Take the (not so) fundamental topic of relative bra size.  Talking to a lingerie retailer it came to my attention that she was not familiar with this concept.  She was aware of sister sized bras but didn’t understand “how they worked.”  Watching the “ahah moment” that my explanation had inspired made me realize just how many knowledge gaps still exist in the lingerie industry.  So with that in mind I decided to share a post on a (not so) fundamental fact regarding bra size.  

The key to understanding bra sizing is to understand that identical mounds of breast tissue are labelled RELATIVE to the ribcage that they are attached to.  The idea in school where you came to believe that an “F” breast was larger than a “B” breast is not necessarily true.  Relative size is at first confusing but regardless of labeling protocols, which globally vary, relative size is the at the root of all sizing methodologies.


Bra size is a description of how a hemispherical mound of breast tissue, with a set overbreast measurement, appears in size relative to the ribcage it is attached to.


The lettered cup sizing system in common use for bra sizing is a method of identifying a relationship between a given volume of breast tissue and a set underbust girth such that IDENTICAL volumes of breast tissue will be labeled as DIFFERENT cup sizes on different sized ribcages.  The following diagram simplifies the concept of relative size.  The red cube is the same size in all three images.  Compared to the cube it is sitting on, however, the top cube is referenced as large in the first image and small in the last image.  If the top boxes represented breasts and the bottom boxes represented ribcages, the first image would be a size 28F, the second image would be a 32D, and the third image would be a 36B.  The top box represents the same volume in all three images, yet the size of the top box, or breast, relative to the bottom box, or ribcage, is different in all three images.  Similarly, breast tissue volume is labeled relative to the ribcage that is it sitting on so an “F” and a “B” sized breast can indeed represent the same volume.

By using relative size the lingerie industry can reuse 26 volumes to offer a full range of cup sizes “AAA up to an R” on band sizes to fit underbust girths ranging from 50cm to 175cm (20 inches to 70 inches).  The reuse of cup volumes makes an otherwise unmanageable size range possible. Without this methodology 20 different cup sizes times 26 different band sizes would create an impossible size range of 520.  While it is true that not all manufacturers offer bras sized in all 20 cup sizes on all 26 bands, with this sizing methodology the potential exists.



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