Bra sister sizes

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

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What does sister size mean?

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

These sister bras 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, 30B, and 34AA do have the same cup volume (they are sister sizes).

See our Size Guide for all topics on measuring »

Are sister sizes the same?

measure bra size
More than one bra size can fit the same woman

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?

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

Maybe your bra band has become too small but you want to keep your cup volume. It could, for example, mean you would go from a 36B to a 38A. Read more about A cup vs B, what’s the real difference »

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Bra sister size chart

I’ll best explain sister bra sizing by giving you an example of the sister sizes of a 38A bra.

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 | going down a band size
  • a bra in 38A has the same cup volume as a bra in 40AA | going up a band size

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

You 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.

There is a very clear video about sister bra sizes from Herroom. Watch the 2-minute video from Herroom »

Discover Thirdlove's ½ cup sizes

What bra size should I then buy?

pichture of 3 bras laying flat about the article bra sister sizing
If you’re in between sizes you might want to try both bras

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.

Have your sizes not changed? Do check the elastic fiber of your bra. It might be time to replace the bra. Read more about how to take care of your bras and how to enjoy them longer »

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.

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 AAA-B cups or in our bra size calculator AAA-O cups
  • 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.

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