sister bras

A cup vs B cup – The difference between these bra cups

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An A cup and a B cup bra can have the same cup volume. How come? Well, you cannot judge a bra size only by its cup size. So, how do they differ?

woman wearing lace pus up bra illustrating article on a cup vs b cup

Maidenform Smoothing Lace-Trim Bra with Push-Up Cups | See

What is the difference between an A cup and a B cup?

Dobreva Floral Lace Everyday Lightly Padded Underwire Bra | A – DD cups | See Dobreva at

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

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How to find the perfect A-cup bra
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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.

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?

To compare different cup sizes you would have to know your band size too. Tips for measuring your band size

Get to know Your Best Bra Size
Use my bra calculator and find out today!

A cup vs B cup could mean:

  • Instead of the 75A bra, you’re wearing right now
  • you might need a 75AA, a 70B, an 80AA, or an 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.

Do you only want to change your band size? It’s your 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?

extreme boost bra a cup b cup

Lily of France Extreme Ego Boost Push-Up Bra A-D cups | See

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

take a good look at your bra in the mirror
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?

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

Handful Women’s Y-Back Maximum Support Racerback Sports Bra with Removable Pads AAA-D | See

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


Because we see ourselves every day in the mirror and we feel our bra all day long. Changing your bra size when needed isn’t something we act upon easily (happened to me). Especially if you wore this size for a long time.

Maybe someone else advised this size on you (and you think they would know…).

  • If you have small breasts be sure to check as size chart that includes all small sizes
  • or use my 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 for you.

Other articles you might like

Measuring guide small cups
The smallest bra sizes online
How to measure small breasts?
About the average US bra size
Small cup large bra band bras
Bra inserts for small breasts
Swimwear shapers

Amazing Bras in 200+ Sizes
30A up to 52J - Shop Herroom Now!

How to find the perfect A-cup bra
Lulalu Bras are Designed Exclusively for Small Boobs