Understanding Bra Sister Sizes

One of my main jobs as a personal shopper is helping people find their perfect bra size. Wearing a bra that actually fits can be a totally transformative experience. Not only does a well-fitting bra eliminate chafing and discomfort, it also makes you look and feel amazing. One thing that comes up in my conversations again and again is bra sister sizes. There seems to be a lot of confusion around what bra sister sizes are and how they work, so I wanted to dispel some of the misinformation and clarify how bra sister sizes can help you get the perfect fit.

Understanding How Bras Are Sized

The crux of sister sizes is that band sizes are more or less absolute , whereas cup sizing is relative to the band size. To demonstrate this, I’ll walk you through how two different hypothetical women with very different measurements can still end up with the same cup size. As we do this, keep in mind that cup sizes are calculated by subtracting the bust size from the band size and increasing in one inch increments. So, a one inch difference between the bust and band size is an A cup, a two-inch difference is a B cup, a three-inch difference is a C cup and so on.

First up, let’s consider a woman—we’ll call her Ashley—with the hypothetical measurements of 30 inches beneath the bust and 32 inches around the bust. Running some quick math on Ashley’s measurements (32-30 = 2), we can easily figure out that Ashley is a B cup.

Next, let’s consider a second woman—we’ll call her Emily—who has hypothetical measurements of 38 inches beneath the bust and 40 inches around the bust. Once again we do some quick subtraction (40-38 = 2) and realize that Emily is also a B cup.

Right now, you’re probably wondering how someone who measures 32 inches around the bust can have the same volume of breast tissue as someone who measures 40 inches around the bust. The answer is that they don’t have the same volume of breast tissue at all. While you may have been taught that the cup volume stays the same even though the band size of the bra changes, this isn’t true. The cup size is simply an expression of the ratio between a band size and a bust size. A B cup with a 30-inch band size is going to contain less volume than a B cup with a 38-inch band size. This is what I meant when I said that cup sizes are relative, not absolute.

If you don’t believe me, the next time you’re shopping for bras, try comparing two bras with the same cup size but vastly different band sizes. The difference in cup volume will be apparent at even a quick glance!

How to Find Your Bra Sister Sizes

Let’s jump into the specifics of finding your bra sister size. Your sister size is found by going down a cup size but up a band size, or

up a cup size and down a band size. Remember that both measurements have to change at the same time in order to maintain that same exact cup volume (only the overall band length changes with sister sizes, not the cup volume).

You can repeat this step several times to get a whole series of sister sizes. However, sister sizes usually work best if you only go one step up or down from your current size — otherwise the band will likely be way too small or way too big. For instance, if your bra size is 36C, you could use a sister size as 34D or 38B.

Why You Need to Know About Bra Sister Sizes

Choosing the right bra size is key to getting a supportive, comfortable bra, and knowing about bra sister sizes is key to choosing the correct bra size. The exact fit of a bra will vary from one brand to another, and even one bra to another depending on the cut and padding. Understanding how to choose the correct size is critical to getting that good fit.

For example, let’s say that you’ve tried on a bra in the size 32C. The band is a bit too tight, but the cups fit perfectly, so you figure that you should go up a band size and get a 34C bra instead. Right? Wrong. A C cup on a 34-inch band bra is going to be larger than a C cup on a 32-inch band size. In fact, a 34C bra size has cups equivalent to a 32D, not a 32C. So if you get a 34C bra, you’ll actually have a band that’s the right size, but cups that are too big, so the bra still won’t fit. Instead, you should get your bra sister size with the larger band, which is a 34B.

I know it can be tough to wrap your head around sister sizes, especially if you were told that cup sizes stay the same volume no matter the band size. If your bra doesn’t fit quite right and you’re not sure how to figure out if you need a sister size or a whole new size altogether, don’t be afraid to reach out. We also have expert consultants standing by to answer your phone calls or chats, and you’re always welcome to schedule an appointment with one of our Personal Shoppers , including myself. I love helping customers find their perfect bra sizes! There’s really no replacement for a comfortable, well-fitting bra.

When to Consider Sister Sizes, and When Not to

Now that you know about sister sizes, you’re probably eager to put this newfound knowledge to work. Sister sizes are a great tool for finding a better fitting bra, but they’re not a one-size-fits-all solution. Here are two scenarios where sister sizes are a great tool—and two more situations where they might not be so helpful:

When your ‘true’ size is out of stock

You’ve found a bra design you love, measured yourself and calculated your current bra size. You’re all ready to order, so you go to put the item in your cart only to find that the item is out of stock in your true size! But don’t give up just yet. See if the store has your sister size in stock instead. Be sure to go up or down a band size based on how bras in your true size tend to fit. For instance, if you usually wear a 32B, but the band runs a little tight, then a 34A might fit you even if it isn’t your “true” size. This method can help you get your favorite bras and shop great bras on sale even if your normal size is out of stock.

When your cups fit, but your band doesn’t

Sometimes your band is a little too tight or loose, but the cups fit perfectly. This is the exact situation that sister sizes are supposed to help you in. Remember that if you change the band size but leave the cup letter the same, you will end up with a cup volume that’s different from what you're used to since the cup sizes are simply ratios. Decide whether you need a band that is smaller or larger, and then select a cup size that is the inverse of what you did for the band. If you’re getting a bigger band, go down a cup size. If you’re getting a smaller band, go up a cup size. 

And finally, there’s two scenarios in which you want to consider a whole new size rather than your sister sizes:

When your band fits, but your cups don’t

Let’s say you have the opposite problem. Your band fits, but the cups don’t. Because sister sizes are meant to keep cup volumes roughly the same, in this case, you actually don’t want a sister size. You want a whole new size altogether. Since the band fits, keep it the same size and then go up or down on the cups accordingly. If you’ve got a 34B bra and it’s too large in the cups, then check out a 34A. If that same 34B bra is too small in the cups, then try out a 34C instead. That will give you a different volume on the cups that should fit you better.

When neither your cups nor your band fit

If both your cups and your band don’t fit, then it’s time to re-measure yourself. Experimenting with sister sizes won’t help you much when neither of the original measurements work for you. Bra size can change relatively fast due to weight gain, monthly hormonal cycles and more. Bras also stretch out over time, so if your bra is pretty old, then it’s probably time that you replaced it. Keep in mind that cup volume also varies between different types of bras , and you might need a larger cup size in a bra with lower-cut cups in order to avoid overflow.

Other Tips for Finding the Right Fitting Bra

At Leonisa, we offer lots of resources to help you figure out what bra type and size to choose and how a bra is supposed to fit. Here are other resources you need to know about, many of them written by us personal shoppers:

Perfect Bra Fit Quiz

This quiz walks you through finding the perfect bra based on what activity you need it for, what level of coverage you like, how much padding you prefer and more. It’s a quick way to navigate Leonisa’s many different product offerings if you want an answer right away and you’re not keen to hop on the phone.

Discover Your Real Bra Size

You can’t find your sister size if you don’t know what your bra size is in the first place. This guide explains how to measure your bust and calculate your correct bra size. It also walks you through the most common mistakes people make when choosing a bra size and explains how to avoid them.

Beyond choosing your bra size, you also have to select which type of bra you want to wear, which presents a whole other dilemma. This breaks down the various benefits that bras can potentially offer and explains what type of bra you’ll need to achieve your desired effect.

How to Wash Delicates, Shapewear and More

Taking proper care of your bras is essential. If they are washed or stored improperly, they can stretch out or become damaged, no longer providing the support you need. This guide explains how to wash and dry your bras, panties and more to make them last as long as possible.

Bra Spillage: What Is It and How to Avoid It

Overflowing cups may be a common bra issue, but it doesn’t make them any less annoying. Learn what your breast shape is and what type of bra is best suited to your chest shape so you can get the perfect bra fit without spillage.

7 Tips for Finding the Best Support Bra for a Large Bust

Having large breasts can make shopping for a bra difficult, especially if you want your bra to look cute while still providing enough support. This article offers seven tips for finding the perfect bra for larger chests.

I hope that this information about bra sister sizes helps you find the perfect fitting bra . Remember that you can always schedule an appointment with me or one of the other personal shoppers at Leonisa if you’d like more guidance on choosing your perfect bra size!