A 32B has the same cup size as a 34A?- How Bra Sizing Works

While it is assumed that nearly all men don't know how bra sizing works, do you know that as many as 80% of women get it wrong too? 

This may come as a shock to you but two cups of the same letter are NOT the same. A 32B and 34B does NOT have the same cup volume. So if you didn't know that a 32B has the same cup volume as a 34A (and a 30C for that matter), then please read on. (Note: I have included a band size conversion chart at the end of this post if inches isn't your thing.)

Wearing the right bra size can (almost) make your boobs look as great as Miranda's

You may know how to calculate your bra size, but it's important to know the "bra matrix" too. That is, how different sizes are related to one another. This is important to know as the fit can differ between manufacturers. Even if you've measured yourself, you might still find yourself wearing a different size for different brands. Knowing how the bra matrix works allows you to quickly figure out what size you need to move up or down to when trying bras. 

Let's start with the golden rule: Cup size is RELATIVE to band size.

 They're both DDs but they definitely do not have the same cup volume! Photo: stanikomania

That's right- you can't just look at the letters. If you usually wear a 32B but there is only a 34B in store, do NOT reach for it thinking it will fit if you clip the band at the tightest hook. They aren't the same! How big a cup volume is depends on the length of the band. Here are a few practical scenarios that you may come across when trying a bra:

Scenario 1: The band fits, cup doesn't

Things are pretty simple if you find the band fits. If you need a bigger cup, just go up a cup size (e.g 32B to 32C): and if you need a smaller cup, go down (e.g to 32A).

Scenario 2: The cup fits, the band doesn't

Now THIS is the complicated part where the bra matrix comes into play.

Rule 1: If you need to go down a band size (make it tighter), you need to go UP a cup size to retain that same volume e.g If you try a 34B and the cup fits but the band is too loose. You should try a 32C for the same cup volume with a tighter band.

Rule 2: If you need to go up a band size (make it looser), you need to go DOWN a cup size to retain that same volume e.g If the 34B fits in the cup but the band is too tight, you should try a 36A.

What does that mean? It means that 36A, 34B, and 32C all have the same cup volume, but with different fitting bands. This equation is known as the 'bra matrix'. So depending on the brand and fit, you may have as many as 3 different sizes in your closet! But they will all fit the same!

Now that you know it's all relative, don't get too caught up on cup size. A lot of bigger women actually need a tighter band and larger cup size but they're just too afraid to face that bigger letter (when really, it's the same cup volume!). Just the same, women with small busts might be afraid to go down a cup and try to squish themselves into a smaller band size just to get that 'B' or 'C'. Don't do it! Wearing the right size will provide your bust the right support and boost it needs. Going up a band size and down a cup size might actually make your chest look bigger than the bigger size!

To reiterate:
– Down a band size, up a cup size
– Up a band size, down a cup size

Silly way I remember this rule is to put it in the context of a bunch of band members with a set number of drinks (cups) at their party. If less people attend than expected, there will be more cups to go around. If, on the other hand, there are more people- there will be less to have.

In a nutshell:
– Down on the band numbers? Up on the cups!
– Up on the band numbers? Down on the cups!

SCENARIO 3: Nothing fits!

Well, it's not rocket science. If both the band and cup are too tight, you need to go up in both e.g 32B to 34B. On the other hand, if it's all too loose, go down e.g 32B to 30A. Keep making adjustments till you find the right fit.

How should a bra fit?

  • A new bra should fit on the LOOSEST (outermost) hook. If you find it's too loose on this hook, go down a band size. As with all clothing, bras will lose their elasticity with wash and wear. The other hooks are there so you can tighten the band as this happens and keep getting a good fit. If you get a new bra that fits only on the tightest hook, you have nowhere to go from there! (Obviously the exception is if you're even smaller than the smallest band size).
  • Cups shouldn't be gaping (if they are, go down a cup size) and neither should your breasts be spilling over the top or sides (go up a cup size if they are).
  • The band should be sitting flat against your skin- it should not be riding up at the back or front. Do not tighten your straps to compensate- get a smaller band instead! The support should come mostly from your band, not your straps. 
  • You should be able to run a finger under the straps and band. It shouldn't be so tight that it leaves red marks on your skin after you take it off.

How to calculate your bra size:

Lastly, I thought I would add the basics just in case some of you don't actually know how to measure yourselves:

Band size:

Using a soft measuring tape, measure your underbust. The tape should be taut against your skin and you should be standing tall and exhaling slightly. Round up if you get a fraction. This number is your underbust measurement.

If your underbust is an even number, add 4 (e.g 28 becomes 32). If it's an odd number, add 5 (e.g 31 becomes 36). This final number is your band size.

Cup size:

Next, measure the fullest part of your bust. (You should be braless or wearing a soft cup bra- no padding!). Don't pull the tape as tight as you did when measuring your underbust- you shouldn't be squishing your boobs! Then, subtract your band size from this reading. If there is no difference or less than one inch difference, you are a AA. If there is 1 inch difference, you are an A. If 2 inches, you are a B, If 3, you are a C. If 4, a D…and so forth!

Band conversion: 












27-28 inches










29-30 inches










31-32 inches










33-34 inches










35-36 inches









Note that some online charts claim that a US and Aussie cup sizes runs one size bigger than UK/French/International sizing e.g a US32A is the same as a Au10AA. Personally, I have not found this to be true at all. I would say a UK32A is an Au10A.

I guess the problem with the bra world is that there really doesn't seem to be an industry standard. You can measure yourself and get your right bra size and something might still not fit in reality. So make sure you understand the bra matrix well so you know how to move up and down within the same brand. And if you're ordering online, then at least you know what size to exchange for if something doesn't fit. Another thing is to always remember to always check out each brand/website's own fitting guide before making a purchase as it can differ from one another (or get yourself measured if you're at a physical store that has a fitting service).

Hope this post was educational for you! 🙂

UPDATE (19/06/2014): I really appreciate all the comments which have come in, but I have decided it is time to close it as this is an old post. If you have a question, I think it's best you ask a professional, or your trusty friend Google. You can also scroll through the old comments and see if there is anything there that might help you. I wish you all the best in your quest to finding the perfect bra! Thanks for taking the time to read this post and I do hope it has been helpful.

Related post: How Do Bra Extenders Work?

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