woman in a lingerie store bra in her hand

A cup vs B cup

What’s the difference between an A cup and B cup? How much do these bra cup sizes really differ?

Triumph Women’s Everyday Bra | AA-B cups See the Triumph collection at Amazon.co.uk


Is B cup bigger than A cup?

You cannot just compare cup sizes, saying cup B is bigger than A cup. Why?

A cup vs B cup
The bras 36B and 38B do not have the same cup volume. However the bras 36B and 38A do have the same cup volume. Going up or down a band size will change the cup volume as well.

For example, maybe your cup size is fine, but you want a larger band of your bra. This could mean you would go from a 34B to a 36A. The 36A bra has the same cup volume as the bra in 34B.

Actually, the cups AAA, AA, and A can be very close together. Read more about the 3 smallest bra cups in our blog.


What happens to the cup volume if I change my bra band size?

woman smiling sitting down
Your underbust and/or bust size may have changed

This happens when you go down a band size

For example, you wear an 85B bra.

You want to go down a band size because you’ve lost some weight. The cups do fit your breasts still nicely.

  • you try 80B and notice the cups are too small.
  • to keep the same cup volume you should try the bra 80C


This happens when you go up a band size

For example, you wear a 70B bra.

You want to go up a band size because you’ve gained some weight. You also feel your cups have become a bit too small for you.

  • you try 75B and find this bra is well-fitted (when going up a band size, so will your cup size). Maybe you should try a 75C as well.

See our Small Bra Guide for more topics on small cups »

Delimira Women’s Lightly Lined Underwire Molded Silicon Convertible Strapless Bra AA-D cups See details at Amazon.co.uk


How will I know my best cup size?

When I measure women and tell some of them they really should wear another bra size they usually are not surprised. How come?

They knew already their bra wasn’t right.

And that’s how it works, doesn’t it? A bad-fitted bra is with you all day and the occasional night. It is NOT a nice feeling having to pull your bra straight again and again.

a cup vs b cup bra
Are you happy with your bra?

My advice is to measure your underbust and bust size. Write the measurements down. If you have a bra fitting that’s fine, but I also suggest you write down your measurements.

Why you should know your bra sizes?

  • Knowing your own sizes will teach you to find your best bra size in a bra size chart or a calculator.
  • You may find it hard to decide on your bra size because you feel you might fit 2 possible sizes. That’s fine. Try both sizes.
  • Different brands may have a different fit. Many women have different bra sizes in their drawer and all of the sizes fit them just fine.
  • Many women wear a cup size that is too big on them. Read more about the average bra size and why I think researches miss out on something.
women with a measuring tape for a bra
Measuring only takes about 10 minutes

A lot of women do fit more than one bra size, especially if you’re just in between two possible band sizes. Read the full post about ‘Bra sister sizing’ here »



Researches say 60 to 80 percent of women do NOT wear their right bra size

Use a bra size chart or try the bra calculator Marks & Spencer |
Go to M&S bra size calculator »