A Cup Vs B Cup – What Is The Difference?

Bras with measuring tape
This post may contain affiliate links. At no cost to you we may earn a commission. See our full disclosure for more info.

There are only minimal differences between A cup vs B cup bras. 

As part of the petite bra sizes, ladies with these bra sizes will tell you that their fit is similar to one another. 

So if you’re asking about their differences, you must be looking to switch bra sizes. And I’m here to help you with that.

Even if chart sizes say that B cups are bigger, there will be instances where A cups actually fit better than B cups. 

Sounds crazy, right?

But thanks to sister sizes, you can find the right bra size for you. 

This applies beyond the A cup vs B cup sizes. And with an understanding of cup sizes, measurements, and sister sizes, choosing the best fitting bra will be an easier choice.

So read on and find out about the difference between A and B cups and how to find out your correct bra size.

Is A Cup Bigger Than B?

Not necessarily. 

There will be instances that an A cup can have the same volume as a B cup bra. But if you look at the standard bra size chart, A cups represent a 1-inch difference between your band and bust size. 

Meanwhile, a B cup represents a 2-inch difference. This usually means you have a bigger bust if you’re a B size.

However, that’s not always the case. 

Have you experienced having a bra that has a loose band but fits your cup well? Or a tight band but again, the cups fit? This is where sister sizes come in.

Sister sizes are bras closely related by cup volume. 

So for example, the sister sizes of a 36F are 38E, 40D, and 42C. Different band sizes but volume really depends on band size, not cup size.

So let’s say right now you’re a 33B. Then you’ve gained a little weight and your band suddenly feels too tight and uncomfortable but your cup size remains the same. You’ll have to look for a sister size for a better fitting bra. 

The general rules are:

  • If the band is too loose and the cup fits: go down a band size, go up a cup size.
  • If the band is too tight and the cup fits: go up a band size, down a cup size.

In this example, you go for a 32A bra size. Now your A and B cup sizes can hold the same breast tissue volume. 

It doesn’t make one cup bigger than the other! 

What Is The Difference Between A And B Cup Size?

Again, when it comes to measuring your bra size, an A cup represents a 1-inch difference between your bust and band, while a B cup represents a 2-inch difference. But between an A and B cup, they only have a 1-inch difference from each other.

Band and Bust Size DifferenceCup Equivalent
Same sizeAA
1-inch differenceA
2-inch differenceB

So you would assume that B cups are bigger than A cups but it’s not always the case. Ladies with smaller breasts may even tell you there isn’t much difference between the two cup sizes. 

You may be a B cup but an A cup bra fits you better. It’s really not as simple. And this is a problem only us women have!

The difference lies more in the band size. 

Changing your band size will change the cup volume as well. So don’t be surprised if you fit into more than one bra size. Bras of the same cup size don’t always fit the same volume. Going back to sister sizes, you can find a better fitting bra with a different cup size even though your breast volume didn’t change.

And when it comes to cup sizes, an A cup size is often identified as the smallest cup size but some brands offer AA, AAA, and even AAAA sizes. 

The range from AAAA to B are considered petite sizes

However, despite the minimal size differences, they still don’t offer the same volume and fit, so you may end up with an ill-fitting bra.

How To Know If You Are A Cup Or B Cup

The surefire way to know if you’re an A or B cup is to get your measurements and try on some bras based on those numbers. And just because you were already wearing an A cup all your life doesn’t mean it’s the best cup size for you.

A lot of us women have been wearing the wrong bra size. 

It’s common, with 60 to 80% of women wearing the incorrect bra size. But since we’re creatures of habit, we stick to what we’ve been used to even if it’s not the best for us (and I’m just talking about bra sizes here, ladies!).

Here’s how to get your accurate bra size measurements. But first, you’ll need a measuring tape, a pen, and paper. 

Writing down your measurements makes it easier to remember and you can bring them along with you whenever you go bra shopping.

  1. Measure your band size
  • Bring the tape around your ribcage just under your breasts. It shouldn’t be too tight or too loose.
  • Round up the figure you get to the nearest whole number (29.2 → 30).
  • If the resulting number is even, add 4. If it’s odd, add 5. So if it’s 30, you get 34. That’s your band size.
  1. Measure your bust size
  • Bring the tape around your bust (at the largest part). Again, it should neither be too tight nor too loose. You can opt to wear a non-padded bra for a more accurate measurement.
  • Round up to the nearest whole number (29.7→ 30). That’s your bust size.
  1. Check your cup size.
  • Subtract the bust size from the band size, then check for your cup size (see the table in the previous section). 
  • From here, you can get your bra size: 34A (band size + cup size).

But it doesn’t end there! As I said, you still have to try on some bras to make sure it fits well. 

Here are a few questions to help you find out if you’re an A or B cup:

  • Does the bra look ill-fitting?
  • Is it comfortable?
  • Is any part too tight or too loose especially at the bands?
  • Can you wear it the whole day without feeling any discomfort?
  • Do the cups support your breasts firmly but comfortably?

Using our example, if a 34A doesn’t fit right, you can check its sister sizes. For this, either you go 32B or 36A. You can also check other brands with extended sizing from AA to AAAA.

Scroll to Top