Sister Sizes: The Bra Secret Every Woman Should Know

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Shopping for clothes is generally a pretty straightforward experience. Sizes become larger as they advance, and that’s that. Bra sizes, on the other hand, are not quite as simple. You have to consider factors such as your unique breast shape, your band size, and your cup size — all of which rarely matches neatly with a single number and letter. This is where sister sizes come in – knowing this can help you find your perfect bra.


What Are Sister Sizes?

Sister sizes are groups of bra-size equivalents that are related by cup volume. For example, a 36F (DDD) cup belongs to the same family as a 38E, a 40D, and a 42C. Stick with us — we’ll explain.

Chances are, you learned somewhere along the line that you had a bra size and if it sort of fit, that was basically the end of the story. You may have also thought that unless one’s breasts actually changed size, there was just no way someone who was, for example, a B cup could also be a D cup. Wrong.

Bra sizes are more like ratios. The volume of a cup size (or how much breast tissue it can hold) varies with the band size. Let us break it down for you.


The Importance of Knowing Your Bra Sister Size

If you’ve ever struggled to measure your bra size and find a bra that fits your unique shape, you’re not alone. It’s no easy feat, and even less so when you’re unfamiliar with your sister sizes. Remember: it’s not you, it’s your bra. Identifying your sister size can be especially helpful if you find that neither your cup or band fit properly. 


Fitting Room Quiz


How to Find Your Bra Equivalent Sizes

So, how do you find your sister size? You can find your two closest sister sizes by (1) going up a band size and down a cup size, or (2) going down a band size and up a cup size. We promise It only sounds more complicated than it is, so we created a handy sister size calculator for you to reference.

Find your current bra size, and your sister sizes are the ones listed in the same row (going across). So, for example, if you’re a 34C, your sister sizes are 30E, 32D, 36B, and 38A. Each row is a family of sister sizes.

bra sister sizes & size equivalents

Let that sink in for a moment: an A cup can also be an E cup. Wild, right?

What’s amazing about sister sizes is that although they can be counterintuitive, they’re key to finding a great-fitting bra. For example, if you’re wearing a 34C and the cups fit, but the band is small, you might think “I’ll just go up a band size and try a 36C.” However, sizing up that way will likely result in cups that are too big because a 36C’s sister size is a 34D. The next sister size to a 34C is a 36B, which is the new size to try instead. The cups will still be the same size, but the band will be roomier. 


how to find your bra sister size

Sister Size Bra Chart

Here’s a breakdown of what to do in a few specific scenarios! Follow along and take a look at our handy dandy sister size bra chart below to visualize going up and down band sizes and cup sizes for your specific size.


Your cups fit, but your band doesn’t: Adjust both your band and cup size in opposite directions. If your band is too small, go up a band size and down a cup size. If your band is too large, go down a band size and up a cup size.

Ex: A 34C cup is the same as a 36B. A 40E(DD) cup is the same as a 42D.

what to do when bra cups fit but band doesn't


Your band fits, but your cups don’t: If the cups feel small, increase cup size. If the cups feel big, decrease cup size.

Ex. If the 32F feels small in the cups, go up to a 32G. If the 32F feels big in the cups, go down to a 32E.

what to do when bra band fits but cups don't

    Neither your cup or band fit: If both your band and your cup are too big, or if they are both too small, then adjust only your band size. Don't worry about changing your cup size - it'll naturally adjust with your band! If your band is too small and your cup is too big, then go up band sizes while reducing cup sizes, and vice versa. This solve is a bit trickier and will require you to try on more bras to find the perfect fit!

    Ex: If a 32D is too small in both band and cup, try a 34D.

    what to do when bra cups and band don't fit


    And sister sizes work with our signature half-cup sizes, too. If you’re a 34B½, you’re also a 36A½, and a 32C½. Because each of those sizes have equivalent cups, the bra you choose depends on which band size feels best to you. Now that you know about sister sizes, you’ll be equipped to choose bras that fit you more precisely.

    bra sister size chart & size equivalents

    Sister Size Up vs Sister Size Down

    Bra sister sizes have the same cup volume, despite having different band sizes and cup letters. The terms sister size up and sister size down might appear perplexing, but it’s actually quite simple. Sister size up means the bra size is one band larger and one cup smaller than the one in reference. Conversely, sister size down is the opposite of that: one band size smaller and one cup size larger. 

    If your bra is fastened on the loosest clasp but still feels too constricting, contemplating your sister size up. It would mean moving one band size larger than your current bra and one letter down your current cup size. For example, if you are currently in a 34D bra size and the band feels snug, you can transition to the sister size up, which is 36C.

    If your bra feels comfortably loose and is already on the tightest hook, you may need to try your sister size down. Go one band size down and one cup letter up. For example, if you wear a 36C and the band feels loose, consider trying a 34D. It should offer a more snug bra band while maintaining the same cup size. 

    Ideally, you should try to sister size up or down with the sister sizes immediate to your current size. Moving further could lead to excessively slack or overly constrictive band size.


    Do Sister Sizes Actually Work?

    Sister bra sizes are a great concept that proposes a perfect fit when your conventional bra size fails to work. But do they genuinely deliver the desired outcome? The simple answer is yes! Sister sizes work well as long as one does not exceed their limits. They can be especially useful when finding a better-fitting bra if your usual size is uncomfortable or unavailable. Bra sister sizes work on the fact that the cup volume is not fixed but relative to the band size. An increase in the band size has a corresponding slight increase in the cup size despite having the same letter. For example, a 34B and a 36B have different cup volumes even though both have B cups.

    Sister sizing will work best when you are going one size up or down. Once beyond that threshold, the bra will change its overall proportion and either be ill-fitted or not provide enough support. It is also worth noting that they do not change the shape or position of your breast; they are only an adjustment of the band and cup size proportions to accommodate your breast optimally.

    They may not be a magic solution, but bra sister size can be a helpful tool that facilitates your quest to find your ideal fit. Get professionally fitted, or try our Fitting Room Quiz to get your best bra recommendation.


    80% of Women are Wearing the Wrong Bra Size

    Did you know that 80% of women are wearing the wrong bra size? You deserve better. If you want some help finding your perfect fit, take our Fitting Room Quiz to find your bra size in just a few minutes, available in A-H, including half-cups!

    Our goal at ThirdLove is to help women find their perfect fit, so don’t keep sister sizes a secret. Tell a friend, coworker, or—of course — your sister.

    With additional reporting by Cathy Bishop and Nicole Yi.