A cup vs B cup what’s the difference between these cup sizes?

This article contains (Amazon) affiliate links, see my full disclosure at the bottom of the page.


An A cup and B cup can have the same cup volume. How come? Well, you cannot judge a bra size only by its cup size. To compare different cup sizes you would have to know the band sizes too. Let me explain.

Check my bra calculator or your shop for bras & size charts

AAA - A cups » Lulalu.com
Fitting Room | video & charts » Herroom
Soft post-surgery bras » Anaono

What is the difference between A cup and B cup?

Dobreva Floral Lace Everyday Lightly Padded Underwire Bra – AA-DD cups | See Dobreva collection at Amazon.com

It’s a question I get a lot from my readers. What’s the real difference between an A cup and a B cup?

And I’m happy they’re asking. Why? Because if you know the answer to this question you really know all there is to know about bra sizing.

AAA, AA, A cups | Shop at lulalu.com

It is very understandable for any woman not to wear a well-fitted bra. Bra sizes can be very close together. The same woman might actually fit more than one bra size. And all these bra sizes may fit her just fine!

So, it may seem cup B is bigger than cup A. But as it comes to bra sizing it’s just not that simple.

Is B cup bigger than A cup?

woman holding measuring tape in article about A cup vs B cup

A cup vs B cup could mean. Instead of the 75A bra you’re wearing right now, you might need a 75AA, a 70B, a 80AA or a 80AAA bra.


Yes. It’s all about the fact that bra sizes can be this close together. A little change in measurements could mean you need a totally new bra size.

There’s also the possibility you might need your bra sister size.

When just changing your band size but wanting to keep the same cup volume it’s a bra sister size you’re looking for.

For example, your bra size is 38C. The bras 36D and 40B all have the same cup volume. These are called your sister sizes

How will I know if I’m an A cup or B cup?

Lily of France Women’s Extreme Ego Boost Push Up Bra A-D cups | See collection Vanity Fair at Amazon.com

I’ve listed some points for you to score yourself. 

put on your bra and have a look in the mirror, how does it look?
does it really look good?
does it feel comfortable?
does its elastics fiber support your breast firmly, but still in a comfortable way?
do you feel your bra the whole day? Not in a nice way…
are you touching your bra a lot during the day (to keep it in place)?
will the bra make your clothes stand out?
do you feel confident wearing your bra?
do you feel womanly?
have your breasts changed after breastfeeding?
have you gained or lost weight?

Sure about your A cup? Have a look at my guide with all best brands for A cup sized ladies

A little unsure about your current bra size? Check out my bra size calculator for AAA-O cups

It could be the elastic fibers of your bra are not strong anymore. So maybe your bra size is still correct, but your bra(s) need to be replaced. Read more on how to handwash your bras so you can enjoy your precious lingerie longer.

Handful Women’s Y-Back Maximum Support Racerback Sports Bra with Removable Pads AAA-D | See details at Amazon.com

When I measure women and tell some of them they should be wearing another bra size they are never surprised!

Why? Because we see ourselves every day in the mirror and we feel our bra all day long. To change your bra size when needed isn’t something we act upon easily (happened to me). Especially if you wore this size for long and maybe someone advised this size on you.

  • If you have small breasts be sure to check as size chart that includes all small sizes
  • or use our bra size calculator. And please play around with the measurements a bit. This will show you whether you might fit more than one bra size. Depending on the model and brand you’ll know the alternatives you have. In the end, you’ll know which size feels best on you.

Other articles you might like

The smallest bra sizes online
Small cup large bra band 40-58
Bra inserts for uneven, saggy and small breasts
Swimwear shapers for a little extra oomph

AAA, AA, A cups | Shop at lulalu.com

Check my bra calculator or your shop for bras & size charts

AAA - A cups » Lulalu.com
Fitting Room | video & charts » Herroom
Soft post-surgery bras » Anaono