Bra Sister Sizes | Why Different Bra Sizes Fit the Same Woman

This article contains (Amazon) affiliate links, see my full disclosure at the bottom of the page.

Sister bra sizes have something in common, but each one of them is also unique. So how are they alike and how do they differ? See the 2-minute video of HerRoom

woman wearing classic t shirt bra thirdlove

24/7^® Classic T-Shirt Bra AA – H cup | ½ cups available | See Thirdlove.com

See my bra calculator or shop for bras at:

Herroom D+ 30% off Sale Comfort Bras
Lulalu Small Bras AAA, AA and A
Amazon Best Sellers in Everyday Bras
60+ sizes & half cups ThirdLove

Topics »

What does sister size mean?

Dobreva Removable Pads Wire-Free Longline Lace Bralette AA – DD cup | See Amazon.com

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.

AAA - AA - A cups | See Lulalu.com

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).

Are sister sizes the same?

woman holding tape for measuring bra sister size

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.

Herroom video on sister bra sizes | 2:05 | see Video

Why you wanna know your sister sizes?

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

your measurements have changed a little, you wonder what size to choose
a certain bra brand is known to have a smaller fit, what size do you then choose
you may wear different bra sizes because your size isn’t always available

For example
✔ you wear 38C and feel the band is too tight, you want to keep the same cup size | you choose 40B

✔ you want to buy a 34A bra, but you read this bra band is a bit smaller than other brands | you choose 36AA

✔ you wear 40AA and can’t get the bra you like in your size | you choose 38A

Bra sister size chart

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

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.

What bra size should I then buy?

If you’re not happy at all with your bra it is wise to check your bra size. 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. Remember that a worn-out bra has lost most of its strength.

There is really 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 my 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.