Let’s face it: bra shopping can be a frustrating experience. One size fits like a dream in one brand, but the same size in another feels too snug or too loose. But before you give up and resign yourself to a life of ill-fitting bras, there’s a secret hack you need to know about—bra sister sizes.

These alternative sizes could completely transform your comfort and confidence.

If you've ever asked yourself, “How do bra sister sizes work?” you’re in the right place. In this guide, we’ll demystify sister sizes, show you when to use them, and help you decode your own bra sister size chart like a pro.

• What Does Sister Size Mean?

• Why Should You Try Sister Sizes?
• Common Situations Where Sister Sizes Are Helpful
• How to Find Your Perfect Bra Sister Size
• Debunking Common Bra Sizing Myths
• Tips for Trying On and Finding the Right Bra Fit

What Does Sister Size Mean?

The concept of “bra sister sizes” might sound like something you'd hear in a fashion think tank, but it's actually a simple, practical strategy for better bra shopping.

Bra sister sizes are alternative sizes that offer the same cup volume but different band sizes. For instance, a 34C and a 36B are sister sizes of a bra. Though the numbers are different, the actual cup volume is nearly the same. Why does this matter? Because not all brands or styles fit exactly the same, and sometimes, your body fluctuates too.

So, instead of settling for a bra that almost fits, you can find your bra size in an equivalent bra sister size that fits like a glove.

Why Should You Try Sister Sizes?

Your body changes. Your favorite styles may be discontinued. Or you just find a bra that almost works. Instead of compromising on comfort, knowing your bra sister sizes gives you flexibility and more options to love.

Here are a few reasons to explore a sister-size bra option the next time you’re out shopping:

Variations between brands

Every lingerie brand has its own design quirks. What fits snugly in one brand might feel loose in another. If you’ve fallen in love with a bra but your usual size feels off, trying a sister size can give you the perfect fit without starting from scratch.

Comfort and support

If your bra leaves marks, causes gaping, or creates that dreaded double-boob effect, a bra cup sister size might be the fix.

Let’s say you’re dealing with cup spillage in your standard size. Instead of sizing up and losing band support, you can try a sister size with a larger cup but smaller band for a snug, supportive fit. Or if the underwire feels incredibly uncomfortable, a sister size could solve that, too.

Finding the right fit

Fit doesn’t just vary between brands: it can be different in different styles of your tried-and-true favorites, too. Sister sizes can offer better alternatives without changing your approach to bra shopping.

Common Situations Where Sister Sizes Are Helpful

Knowing how to pivot to a sister size can be a total game-changer when you’re dealing with common fit issues. 

One of the most frequent problems is with the band. Since the band provides about 80% of your bra’s support, getting the right fit here is crucial. If your band feels too tight, the fix might be as simple as going up a band size and down a cup size. 

For example, if you usually wear a 34C and it’s digging in, try a 36B instead. On the flip side, if the band feels too loose and isn’t offering enough support, try going down a band size and up a cup size, like switching from a 36C to a 34D.

Cup issues are another area where sister sizes come in handy. If you’re dealing with overflow, where your breasts spill over the top or sides of the cups, try a larger cup size with a smaller band to maintain support while gaining more room in the cup. 

If you’re noticing gaping or wrinkling in the cups, a smaller cup with a larger band might provide a better fit. The key is maintaining the same cup volume while adjusting the band for comfort and support. 

How to Find Your Perfect Bra Sister Size

Finding your perfect fit isn’t just about guessing your bra size—it’s about understanding your measurements and using a bra sister size chart to your advantage. It all starts with two simple measurements: your band and your bust.

To measure your band size, wrap a soft measuring tape snugly around your ribcage, just under your bust. Make sure the tape is level all the way around. Take the measurement in inches and round it to the nearest whole number.

Next, measure your bust by wrapping the measuring tape around the fullest part of your chest, usually across the nipples. Again, keep the tape comfortably snug and level. Round this number to the nearest whole inch. To determine your cup size, subtract your band size (the number you calculated, not the raw underbust measurement) from your bust measurement. Each inch of difference represents a cup size, so 1 inch equals an A cup, 2 inches equals a B cup, 3 inches equals a C cup, and so on. 

For your bra sister sizes, the cup and band size will both change and have an inverse relationship. For example, if you go down a band size, you’ll go up in cup size and vice-versa.

Debunking Common Bra Sizing Myths

Let’s set the record straight on a few widespread bra misunderstandings:

Myth: "If you go up in band size, your cup size stays the same."

Truth: Nope! Cup size is relative to band size. A 34C and a 36C don’t have the same cup volume. That’s why sister sizes are so important.

Myth: "A bigger cup always means a bigger bust."

Truth: Cup size alone doesn’t determine how big your bust looks. A 32D has a smaller cup than a 38B. Surprising, right?

Myth: "You should wear the same bra size in every brand."

Truth: In a perfect world, yes. But reality calls for flexibility, and that’s where bra size sister sizes come to the rescue. Use a size chart whenever you shop to find the perfect fit.

Tips for Trying On and Finding the Right Bra Fit

Once you’ve got your size and sister sizes in hand, here’s how to find “the one”:

1. Start on the loosest hook. Your band should feel snug but not restrictive. A new bra should fit best on the loosest hook so you can tighten it as it stretches with wear.

2. Check that the center gore, the part of the bra between the cups, should lie flat against your sternum. If it floats, try a different size or bra style altogether. Browse Soma’s types of bras to find the perfect one for your shape.

3. Watch the cups to make sure there’s no gapping or spillage. The cups should hug your bust gently without cutting in or floating away.

4. Move around by raising your arms, twisting, and sitting. Does everything stay put? That’s a good sign.

5. Consider the fabric and style of the bra. Some fabrics have more stretch, and some bra styles (like spacer bras) are made for breathable comfort. Don’t just rely on size. It’s important to look at the structure too.

At the end of the day, your bra should make you feel supported, confident, and comfortable, whether you’re heading to work or choosing something comfortable enough to sleep in.

The magic of bra sister sizes is that they give you more options, so you don’t have to settle for a “pretty good” fit when the perfect match is just a size swap away.

Shop our bras to find new styles that offer comfort, convenience, and versatility.