Understanding Bra Sizes...

Understanding Bra Sizes...

Bra buying and fitting can be a bit of a minefield, not only because all brands are different but also because lots of people don’t actually understand bra sizes and what they mean. Do not worry though, that is where we come in as your fairy boob mothers to help with everything boobs!

To begin to understand bra sizes I think we need to address that the sizes look a little weird, for example:


A strange mixture of both numbers and letters, it's no wonder people get confused. So what do they mean? 

The Numbers:

Well let's start with the numbers - they represent your band size, or the size of your body around your ribcage. They only come in even numbers and progress like this:

6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26

It’s thought that Australian sizes derive from dress sizes and if you wear a size 12 in clothing you should wear a size 12 bra band size, however this isn’t a particularly accurate way to size as so many women’s bodies are different. 

In bra fitting it is essential to find the correctly fitting back band before you work out any other measurements therefore this is how you do it:

Take a measuring tape and measure your underbust (this is horizontally around your ribcage underneath your boobs) do make sure to hold the tape firmly with no slack. Now you can take this measurement in either cms or inches, what you measure here indicates your band measurement and this can be converted into an Australian band size below: 

Underbust in cms

Underbust in Inches (UK sizes)

Australian Sizes


































Getting this right is the key to a correctly fitting bra, the band also holds 80% of the weight of your breasts so remember that the back band must be firm and shouldn’t move around. 

The Letter:

Now that you’ve got the correctly fitting band you need to find your cup size. This is where the letter comes in. The letter refers to the size of the cup in your bra and measures the volume of breast tissue it holds. The sizes go as follows:

A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K

What we need to understand about cups is that they are not a definitive size, for example not all F cups are the same. To know the actual volume of a F cup we need to know the back size it is teamed with as this will affect its actual size. Below is an example of how cup size volume increases when you increase the back size. 

10F  ---> 12F --->  14F

Each of these cup sizes is one cups worth of volume larger than the last as they have increased one back size. So a 12F is one cups worth of volume larger than a 10F. 

This is why we can never know how big an ‘F’ cup is until we know the back size it is teamed with.

Sister Sizing or ‘Swing Sizing’:

This then nicely leads us on to sister sizing. As the same letter cup size (e.g. F) increases with every back size, it also means that in order to keep the cup volume the same we need to change the letter that goes with it. 


10G  ---> 12FF --->  14F

Each of these cup sizes are the same volume, all that is changing is the band size it is teamed with.

If you took the cup from the 10G and placed this over the boobs of a lady who wears a 14F they would fit the same but the back would be too tight. 

It can be really useful to know your sister-size as some brands vary and if your body changes but your boobs stay the same you can go up or down in the band size without compromising on your cup fit. 

So there we have it, bra sizes aren’t that confusing are they? 

Team Curvy xoxo

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