Bra sizing explained: How does bra sizing actually work?

Bra sizing explained: How does bra sizing actually work?

(3 minute read)



Do you fall in to majority of women in Australia that don't *truly* understand how bra sizing works? 🤷🏽‍♀️ There are three main points to wrap your head around when it comes to understanding bra sizing. But the truth is that if you don't know all three comprehensively then you may as well not know any of it 😆

So here goes... (Make sure to read until the end when it all comes together!)


1. What does the number on a bra size mean?

The number or the 'band size' is a representation of your ribcage girth. We get this size by taking a measurement around your ribcage, about where the bra band sits. This measurement is taken in inches. In the UK they use the inch measurement on their bra sizes. Eg if you are 32" then you are a size 32. In Australia we translate that to a size. In this example the size 32" converts to a size 10. A measurement of 34" converts to a size 12... A measurement of 36" converts to a size 14 etc etc. This 'number' should correlate somewhat to your 't-shirt size'. A common misconception with the number component of the bra size is that is correlates to shoulder width 🙅🏽‍♀️ As mentioned, this number gives us a guide to what size bra we need to accomodate the circumference of your ribcage, overlying skin and subcutaneous tissue.

2. What does the letter on a bra size mean?

The letter component is an indiction of cup volume. The amount of actual breast tissue/fat in volume. The biggest misconception we come across is that the cup size, "the letter" eg a C cup or a G cup is a set unit of measurement. It is not. A 10B is a completely different cup size to a 16B. They are 4 cup sizes different 🤯 That's because the cup volume is proportional to the ribcage/band size. Read below to wrap your head around this... 


3. How 'the Number' and 'the Letter' work together on a bra size

Understanding this is where the magic will happen for you when out bra shopping solo 💁🏽‍♀️ Have you heard of sister sizing? That's what all of this is about... The band size will dictate how much cup volume your 'cup size' has. So if someone says they are a 'D cup' it doesn't give us any information about their breast volume unless we know their band size.


bra sizes sister sizes 

As you can see by the image above both of these sizes are "D cups". But the 24D breasts are roughly the size of melons yet the 8D breasts are only the size of small apples 😵

For each increase in band size, the cup volume increases one 'size' 🙌🏽

Likewise for each decrease in band size, the cup volume decreases one 'size'.


This is where sister sizing comes in to it...

If someone was commonly wearing a 12DD however was finding the fit of the band a bit loose then they should try the 10E. This is the sister size. By shifting to the 10E they will make the band size a size smaller but maintain the same cup size.

Another example would be a size 16F lady finding the band size too firm. If she was to go to a size 18F then she would increase the band size successfully but she would also be adding a size to the cup unnecessary. The move she needs to make is to a size 18E. The 18E will be a size bigger in the band than the 16F, but the cup size will be the same as the 16F.  In this example we went up in the band size and down in the cup to find a better 'sister size' 💁🏽‍♀️


How to determine your bra size

Here's a page on our website that goes in to more detail about how to measure your bra size.

Note: You can search by bra size on our website to view our comprehensive range.



Tish bra fitter melbourne
She Science, our Specialty Bra Store offers cup sizes A - J and band sizes 6 - 30. We work with 16 industry leading brands from all over the world. Our comprehensive Sports and Everyday Bra fitting service is available by appointment, out of our fitting suite in Camberwell, Melbourne (Australia). Come on in and we'll find you your best match taking in to account your size, shape, activity demands and personal style preferences. Our team of experienced bra fitters would love to meet you.
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