Bra sister sizes

As for any woman, we all might fit more than one bra (sister) size.

  • What does sister size mean?
  • Are sister sizes the same?
  • Bra sister size chart

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What does sister size mean?

Apart from your current bra size, you too have bra sister sizes.

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These sister sizes are bras with different numbers, but all have the same cup volume.

Remember that when going up or down a band size your cup volume will automatically change as well. For example, the bras 32A and 34A do not have the same cup volume. The bras 32A and 34AA do have the same cup volume (they are sister sizes).

Read more about how to take your measurements »

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Are sister sizes the same?

Sister sizes have the same cup volume even though their cup size and band size both have different numbers. For example, the sister sizes of a 36B bra are 34C and 38A.

Why you wanna know your bra sister sizes?

  • your band feels too wide but are fine with the cups
  • your band feels too tight but are fine with the cups

Knowing about sister sizes makes it easy for you to choose an alternative size if you need one


Bra sister size chart

I’ll best explain sister bra sizing by giving you an example. Or you might wanna watch the video about sister bra sizes from Herroom. Watch the 2-minute video from Herroom ».

sister size bra chart
A sister size bra chart

For example:

  • a bra in 38A has the same cup volume as a bra in 36B
  • a bra in 38A has the same cup volume as a bra in 40AA

You can see when going up a band size your cup size volume will automatically be bigger as well. To keep the same cup volume you will have to go down a cup size.

And when going down a band size your cup size will automatically have less volume. To keep the same cup volume you will have to go up a cup size

bra size chart with an example of a sister size
Bras 34B and 36A have the same cup volume | source Thirdlove.com


If you’re not happy at all with your bra it is wise to check your underbust and bust. It could well be that you might need a different bra size, to begin with.

Reasons our body and therefore our breasts can change could be of causes like aging, an operation, losing or gaining weight, having smaller breasts after breastfeeding. Regularly taking your measurements might be needed.


The collection bras of ThirdLove come in cups AA – I. They exclusively have ½ cup-sized bras in their collection | See collection AA – I cups, including ½ cups (ThirdLove)


What bra size should I then buy?

More than one bra size may fit you just fine

There really is only one way to find out what your best bra size is.

  • measure your cup size and band size
  • check your sizes in a bra size chart or bra size calculator
  • dare to try different sizes, try if needed your bra sister sizes

If you have small boobs you really need a small bra size chart (or our bra size calculator) to check your size. I see too many women settle for an A or B cup, but should be wearing an AA or AAA cup instead. If you check a chart that starts with cup A, you won’t be able to check your bra sizes. Simply because your bra size may just not be in it.


Go to bra size calculator and determine your best bra size


Amazon AA cup bras »


Amazon A cup bras »


Amazon B cup bras »


Amazon C cup bras »


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