Understanding Bra Sister Sizing: What it is and how to use It

Let’s face it: finding a bra that comfortably fits can feel like hunting for buried treasure—complete with a map that’s half-burned. Understanding “sister sizing” can be a huge help, but too often it’s not well-understood or its misused. Time to clear the fog and settle this so you don't chase false leads. 

 

Overused? Yes it is. 

Misunderstood? Yes it is.

The secret magic bullet? No it is not. 

Sometimes helpful? You bet. 

 

The most important thing to know is that sister sizes affect the band fit only. You use a sister size when you have a band fit issue, not for a cup fit issue.

 

If your sister size works, it's likely because you were wearing the wrong bra size to begin with, but that's not a popular idea so it's never said out loud. 

 

What’s the Deal with Bra Sister Sizing?

Bra sizes can look different on the tag but secretly share the same DNA, well sort of. That’s where the confusion starts. It’s all about the relationship between band size and cup size. When you move up or down in band size, you do the opposite for the cup size and—voilà!—the cup volume stays the same. The key word is “volume” (lots more on that below).

 

Cracking the Sister Size Code

If you can remember a simple game of opposites, you’re golden. It’s very simple:

Band down – cup up

Band up – cup down

 

Time to play bra sleuth! Let’s use a 36D and let’s map out its sisters:

Smaller band, same cup volume: 32DDD/F and 34DD/E

Bigger band, same cup volume: 38C and 40B

 

QUIZ (Answers below)

a) What are sister sizes for 32B?

b) What is the bigger band sister of 38DD?

c) What is the smaller band sister of 44B?

d) Are you totally freakin’ cool?

 

Answers: a) 30C and 34A        b) 40D             c) 42C              d) yes

 

Grab your current size and play the same game. What would be your sister going down a band size? And up a band size? Remember to use opposites and the cup volume and fit will remain the same.

 

OK, but Now What?

Theory’s fun, but how do you actually use this? Maybe your current bra is too tight, too loose, or you gain and lose weight in specific areas. With sister sizing in your arsenal, you can fine-tune your fit without sacrificing the great support you already found. Well, maybe. 

 

It’s your go-to when:

• The cups fit great but the band is too tight
  • You’re being suffocated and you can’t loosen more
  • Sister size to a larger band size (and down a cup)

• The cups fit great but the band is too loose
  • It’s riding up, falling down or straps slip
  • Sister size to a smaller band (and up a cup)

• Your weight fluctuates during your cycle but your breasts don’t change much:
  • Boobs are fine, back is not
  • Depending on how much you fluctuate, having up to three sister sizes can help balance out the fit. So if you’re a 38C in the middle of your range, then also have a 36D if you shrink and a 40B if you expand

It’s NOT your go-to when:

• Your size is out of stock: the band is part of the fit and can’t be swapped interchangeably
• You don’t fully understand: Mistaking sister sizes for twin sizes is a bad thing
• You need a larger or smaller cup size: Cup fit doesn’t really change in sister sizes
• Your bra doesn’t fit and you don’t know why: Blind experimentation creates frustration
• Your breasts fluctuate in size throughout your cycle: You need bras with varied volume, too
• Your friend or the internet told you to: See above on “understanding”

 

Why does sister sizing work? (Optional read from here down)

When it comes to bras, cup volume is the quiet topic that doesn’t always get the spotlight it deserves. You might think a “C cup” is a universal standard, and C cup is a certain size of breasts. But here’s the complication: cup letter sizing isn’t absolute. You’ve probably heard (or think) that D cups are “big” and A cups are “little.” That was true long ago in bra history, but once specific band sizes entered the market, that concept of size went out the window. Cup size is a sneaky little variable that shifts depending on the band size it’s paired with—and understanding this can totally level up your bra game.

 

What Is Cup Volume, Anyway?

Cup volume is simply the amount of space inside the bra cup—how much “room” it has for your breast tissue. It’s what determines whether your girls are cozy, cramped, or swimming in too much room for activities. A 32C and a 38C? Not the same cup volume / boob size, not even close really.

 

How Band Size Messes with Cup Volume

The secret is in the relationship between band and cup. Bra sizing is relative, not absolute. Traditionally, the cup letter measures the difference between your underbust (band) measurement and your bust measurement:

A 1-inch difference = A cup

A 2-inch difference = B cup

A 3-inch difference = C cup

And so on.

 

But here’s the kicker: as the band size increases, that same “difference” translates to a bigger overall volume. Why? Because a larger band stretches the cup across a wider frame, making the cup physically bigger.

 

For example:

A 32C (32-inch band, 35-inch bust) has a 3-inch difference, but the cup is designed for a smaller ribcage, so it’s compact.

 

A 38C (38-inch band, 41-inch bust) also has a 3-inch difference, but the cup stretches across a broader chest, so it holds more volume.

 

So, while both are “C cups,” the 38C could practically swallow the 32C’s cup volume whole. In other words, your C cup (or whatever you happen to be) is not only not the same C cup as your huskier bestie, your C cup is actually the same as your sister size D or even DD cup! Wild, right?

 

It can't be said enough: cup size alone means absolutely nothing. Understanding your own breasts in terms of a letter by itself leaves so much left unaccounted for. To fit your boobs with a bra, you have to understand them in terms of their volume. Cup alone is like saying you drink one glass of wine a night, but everyone has seen the gigantic wine glass…so there’s no way to know how much wine you drink unless you answer in ounces—volume! And the volume of a bra’s cups is determined by the band-cup relationship.

 

Sister Sizing and Keeping Cup Volume in Check

All this is how and why sister sizing can actually help. Since cup volume changes with band size, sister sizes are the workaround to keep that volume consistent when you tweak the fit in the band. 

 

Understanding sister sizing looking via volume:

A 34C has a certain cup volume—say, enough to hold a juicy orange (keeping it fun here).

 

Step up to a 36B: the band’s longer, but the cup’s smaller relative to the band, so it still cradles that same orange.

 

Slide down to a 32D: tighter band, bigger cup letter, but—yep—same orange-sized volume.

 

The magic happens because the cup volume stays constant across these sizes, even though the band and letter combo changes. 

 

The Real-World Twist

Here’s the fine print: cup volume isn’t perfectly identical across sister sizes. The shape of the cup, the stretch of the fabric, and your unique curves can affect how that volume feels. A 36B might distribute the volume a smidge wider than a 34C, which could matter if your breasts are fuller on top or bottom. If you keep sister sizing in the fine tuning part of your bra fit journey, it's far more likely to be the genuine help it's so often billed to be. 

 

Also, using sister sizes is safest and most linear when sticking with the same style and brand. Chaos can ensue when you sister size with a different brand and/or style. 

 

Hope this helps make sister sizes slightly more usable and understandable!

 

 

As always, if you have questions about this topic or finding your own best bra fit, reach out to Billy here or at billysbras@gmail.com