woman in a lingerie store bra in her hand

A cup vs B cup

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

triump bra cup a and cup b

Triumph Women’s Everyday Bra | AA-B cups See Amazon

Is B cup bigger than A cup?

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

See my bra calculator or visit

Bras and Size charts
M&S UK | M&S IR | M&S US | First Bras

A cup vs B cup
The bras 36B and 38B do not have the same cup volume. However, 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 34B to 36A. The 36A bra has the same cup volume as the bra 34B.

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

Changing your bra band size

dobreva bra cup a b

DOBREVA Women’s Lace Non-Padded Bra Plunge See Through | See Amazon.co.uk

When you go down a band size

For example, you wear the 85B bra size.

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

When you go up a band size

For example, you wear the 70B bra size.

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.

Am I A or B cup?

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

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.

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 different fits. Many women have different bra sizes in their drawers and all of the sizes fit them just fine.
  • Many women wear a cup size that is too big for them. Read more about the average bra size and why I think researchers miss out on something.
women with a measuring tape for a bra
Measuring only takes about 10 minutes

Other articles you might like

What is considered a small bra size?
Teen bras, buying your first bra
Triumph bras aa cups
The best bikini’s for small busts
10 Plunge bras
Tops with built-in shelf bras
Small Bra Guide

See my bra calculator or visit

Bras and Size charts
M&S UK | M&S IR | M&S US | First Bras