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

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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?

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What does sister size mean?

At its core, bra sister sizing is the practice of finding a bra that fits you well, even if it’s not your “standard” size.

The idea is to identify a bra with a different band and cup size combination that still provides the same level of support and coverage.

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This allows you to explore a wider range of bras, rather than being limited to a narrow selection of your exact size.

For example, let’s say your typical bra size is 34B. Using the principles of sister sizing, you could potentially find a comfortable, well-fitting bra in sizes like 32C or 36A.

The key is that these “sister” sizes share the same cup volume, even though the band and cup sizes are different.

So, apart from your current bra size, you too have bra sister sizes. These bras have different numbers, but all have the same cup volume.

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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).

Why you wanna know your sister sizes?

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

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

The beauty of bra sister sizing is that it allows you to tailor the fit of your bra to your specific needs.

Get to know Your Best Bra Size
Use my bra calculator and find out today!

If you find that your standard size is a bit too tight in the band, you can go up a band size and down a cup size to maintain the same overall fit. Conversely, if the band feels too loose, you can go down a band size and up a cup size to achieve the perfect level of support and coverage.

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.

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.

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

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.

Do you need some extra bandwidth? Adding a bra extender helps you wear your bras with comfort again.

There 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 my bra size calculator
dare to try different sizes, if needed your bra sister sizes

If you have small boobs you 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.