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Bra Sister Sizes + Sister Size Chart

Ever wonder why bras come in so many different sizes? It’s because when choosing a bra, you have to consider factors like the cut and style, your torso and cup size and even your unique breast shape. Unlike other clothing, bra sizes rarely fit across the board and often a cup or band size can fit differently by brand or label. Luckily, there are bra sister sizes that you can use to find the perfect fit every time. 

What are Bra Sister Sizes?

Illustrations of a white, pink and gold bra representing bra sister sizes of 34C, 32D and 36B

Bra sister sizes are groups of bra size equivalents where the cup size, or volume is consistent, but the length of the band around varies. For example, a size 34B is in the same sister size family as a size 32C as well as a size 36 A. Think of it as a waist and inseam measurement! You may be a 28” in jeans, but depending on your height, would want a shorter or taller inseam! 

We’ll break it down for you. 

When it comes to bra sizes, the volume of a cup size (or how much breast tissue it can hold) varies with the band size. In sister sizes, as the band’s width goes up, the cup size must come down to maintain the ratio of how much breast tissue the cup can hold. For example, if you’re a 34C, and your cup size feels great, but your band’s a bit snug, your sister size up would be a 36B.

By using bra sizes as ratios, you can bump up or down from your “regular cup” to find a sister size that is most compatible with your body’s unique proportions.

The Importance of Knowing Your Bra Sister Size

3 illustrations representing why you should know your bra sister size including to fit your breast shape and make shopping easier

Knowing your sister size off-hand can be very helpful when shopping as you can easily grab a few different sizes to bring into the dressing room. Better yet, you won’t need to measure your bra size as often, and finding a bra that fits your unique shape will be easier than before. 

That being said, knowing your accurate measurements is key to getting the best fit, but your particular breast shape may affect the way certain types of bras fit and feel. That’s where sister sizes come in as you can easily navigate what size might work better. What matters most is that the bra size you ultimately choose is comfortable and gives you maximum support—the most important perk of knowing your sister size. 

How to Calculate Your Sister Size 

5 step list explaining how to calculate your sister size accompanied by a photo of a woman clasping a black bra

So, how do sister sizes work? Learning how to calculate your bra sister size is easier than it may seem. We’ll walk you through our sister size calculator. 

1. Find Your Current Bra Size

The first step to finding your bra sister size is knowing your current bra size. If you don’t know your band and cup size or have never properly learned how to measure bra size, you’ll want to start there to get your most accurate measurement. 

2. Decide Whether You Should Go Up or Down in Sister Size

Once you have your current bra size and notice that the fit is either too small or too large, you can decide whether your cup size or band size should change.

Cup Size

If your cup size needs to increase, you can easily bump up one cup size for more volume. For example, moving from a 34B to a 34C. A good indicator of this is whether your breast tissue is spilling out of your cups. If you think you need to go down a cup size, look for indicators like gaps in your cups, or if they are not lying smoothly on your breasts.

Band Size

If your band size needs to change, then first try your larger or smaller sister size. If your band is too tight, go up a band size and down a cup size. For example, going from a 32C to a 34B. If your band is too loose then go down a band size and up a cup size. For example, going from a 34B to a 32C.

Re-evaluate to Find the Perfect Fit

Once you’ve tried your larger or smaller sister size, re-evaluate to ensure it fits your unique breast shape and body perfectly. To help you remember your sister size at the drop of a hat, we created a bra sister size calculator that’s a breeze to reference. 

Bra Sister Size Chart 

In order to use our bra sister size chart, start by finding your current bra size, and your sister sizes are listed in the same row across. This easy-to-use bra size chart offers a different family of sister sizes per row with larger band sizes on the right and smaller band sizes to the left.

For example, if you’re a size 36D, your sister sizes are size 34DD or 38C. 

Band Size

Cup Volume

30

32

34

36

38

40

42

44

46

48

30A

30B

32A

34AA

30C

32B

34A

30D

32C

34B

36A

30DD (E)

32D

34C

36B

38A

30DDD (F)

32DD (E)

34D

36C

38B

40A

30DDDD (G)

32DDD (F)

34DD (E)

36D

38C

40B

42A

32DDDD (G)

34DDD (F)

36DD (E)

38D

40C

42B

44A

32H

34DDDD (G)

36DDD (F)

38DD (E)

40D

42C

44B

34H

36DDDD (G)

38DDD (F)

40DD (E)

42D

44C

46B

36H

38DDDD (G)

40DDD (F)

42DD (E)

44D

46C

48B

38H

40DDDD (G)

42DDD (F)

44DD (E)

46D

48C

38I

40H

42DDDD (G)

44DDD (F)

46DD (E)

48D

NOTE: Not all Sister Sizes shown will be available at Tommy John.

It’s easier than it seems to find your sister sizes but it might leave you scratching your head and wondering if you could really fit into your sister cup size. 

That’s because when we think of shopping for clothing we think sizes progress linearly (small, medium and large) and if a size doesn’t quite fit we have to bump up to the next full-size up. That’s the wonderful thing about sister sizes. 

Here’s an example of sister sizing:

If you’re trying on a 34C and the cups fit, but the band is too small, you might automatically reach for a 34D. However, sizing up that way will likely result in cups that are too big. 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 give you the room you need. 

Should You Wear Your Sister Size?

Back hook section of a bra with arrows pointing out what your bra hooks say about your bra sister size

The answer isn't always "yes'' for every body type or for every bra. Bra fits can vary depending upon the style, design and brand. We are here to help you decide whether you should wear your sister size, based on the following questions.

Which hook setting do you use to fasten your bra? 

If you can hook your bra on the tightest hook then you most likely should try your sister size down (down a band size, up a cup size). 

If you can hook your bra on the loosest hook, then you most likely should try your sister size up (up a band size, down a cup size). 

If you can hook your bra on the middle hook, then you're works done as you're in the perfect fit!

How do your band and cups fit?

If your band fits, but your cups don't: If the cups feel small, go up a cup size. If the cups feel big, go down a cup size.

If your cups fit, but your band doesn’t: Adjust both your band and cup size by using your sister size. Remember: if you go up a band size, you need to go down a cup size, and vice versa. 

If neither your cup nor your band fits: Go down a band size if they’re both too big, or go up a band size if they’re both too small. 

Bra Sister Size FAQS

Sister sizes to some might seem like a well-kept secret but to others, it’s the best way to ensure you are finding the best bra fit for your body. We want all women to find a bra that has their back so we’ve debunked frequently asked bra sister size questions below.

Do sister sizes really work?

Sister sizes work well but you most likely will only be able to move up or down 1-2  sister sizes. Moving anymore most likely will change the entire fit of the bra and it will either be too small or too large. 

Is a DDD the same as an F?

It might seem confusing, but yes a DDD is the same as an F—at times they are labeled differently based on the brand. After size D you can either go up to size DD which is the bra size equivalent to size E, size DDD which is equivalent to size F, and size DDDD which is equivalent to size G. 

Our bra sister size chart above can tell you your sister size all the way to bra size 38I. 

Are 34B and 32C the same?

In terms of sister sizes, yes size 34B and size 32C is bra size equivalents. 34B and 32C are the same exact cup sizes, the only thing that changes is the length of the band.

Gone are the days where if your size is out of stock, you have to turn to a bra alternative or even go braless. No body or breast shape is the same, so it only makes sense to have a few sister sizes that you can try to find the perfect bra fit every time. 

Find your true or sister size in any of our bra or bralettes or reach out to one of our bra fit experts, who are always available on live chat, or by phone to guide you through the proper fit process.