What a Cup Size Says, And Doesn't Say, About The Size of Your Boobs

December 30 2021 – Athena Kasvikis

What a Cup Size Says, And Doesn't Say, About The Size of Your Boobs | Behave Bras
What a Cup Size Says, And Doesn't Say, About The Size of Your Boobs | Behave Bras

Cup sizing in bras sounds pretty straightforward. 

It follows letters of the alphabet, and yes, it goes above the letter “D” and can go all of the way up to a Z (we’ll get into the alphabet and cup sizing in a different post.)

So cup size should be straightforward, right? I’ve heard many many women say things like: “Oh, I was a C cup when I got married” or “I’ve been a DD since I was in high school”. 

Just talking about the one part of your size, the cup, can be decieving for understanding bra sizing better.

It's a tricky proposition that doesn’t tell us how large your breasts actually are. That's because our cup size is not JUST the volume of our breast(s), but instead is proportional to our band size. 

And to get a cup that fits, you'll need to know what the difference is between the measurement of your breasts vs the measurement of your band. 

Now. I know this idea of measuring one’s self can get some of you ladies a bit huffy. 

The last thing many of us want, in the middle of a pandemic, is to know the exact number of inches around our bands are vs what they were months ago. 

It's easier to continue to think that the egregious amount of homemade pizza I’ve been eating has not gone to my waistline when I don't have a new number to compare to.

But alas, we must have some measurements to get started for better understanding of our busts. 

Because the letter of your bra size doesn’t mean as much unless we know what the band is. 

This is because all D cups (all cup sizes in general) are proportional to one’s band. Your general cup size is more of a measure of what I call your “boob projection.” It’s the differential of your breast to band that is an approximation of the volume of your cup. 

My G cups sound kinda massive in theory, until you realize that my band is a 32 (at least it was before the pandemic pizza binge. Maybe a 34 now), and so they’re really not that large in the grand scheme of breasts on the alphabet. 

When you see the 32 band, you know that what I’m really saying is that I have approximately a 7-8 inch breast-to-band differential. Likely I’m measuring in the high 30’s on the bust and a 28-29 inch on the band. The difference comes in around 7-8 inches. Hence, the starting point of a G cup. Check out our sizing chart for more info on this.

What it’s absolutely NOT telling you is that my G cup is the same size and volume as a 38G cup.

That would be far too easy. 

Why is this a false equivalency in actual size when both cups are a G?

When someone has a differential of 7-8 inches, it’s only telling you their boob projection. It’s NOT telling you their individual breast circumference or the fullness of the breast. It’s not giving you a measure of the weight and size of each breast, just the difference vs the band. 

So if we compare a 32G to a 38G: we have more inches on the band in a 38G so we need more boob if we’re going to still have a 7-8 inch difference. Essentially, the 38G breast volume is going to be significantly bigger than the 32G breast volume to achieve such a large differential. 

Something that’s helpful in bra shopping when we’re looking to find the right fit is called Sister Sizing.

Essentially, Sister Sizing gives you an idea of where to look for a new size when your cup, band, or both, changes vs where you started from.

Here’s a handy chart from that I found to showcase this:

Let’s say you’ve lost weight and your band is loose (less pizza maybe?) but your breasts are still feeling the same size. You would use your starting size (let’s use 32D) and go down a band to a 30. To keep the same cup fit, we have to go up a cup to an E. 

A 30E bra would be a sister size to a 32D in general. You go 1 size down in the band, you go one size up in the cup. You move diagonally on the charts above because you’re looking for the same cup/band ratio.

Let’s now say that you’ve pounded pizza for 3 months straight and you’ve gained some weight in the band but your boobs haven't really changed. 

You’d look for one band size up, so a 34 inch band, but would need 1 cup size down, so an E cup, to stay in a similar cup.


If all that pizza went to your boobs AND your band, you may look for a 34 inch band in a cup size larger as well and land on an F cup.

Are you starting to see why “I used to be a C cup” is kind of an ambiguous indicator of someone’s actual cup size now? This person could’ve very easily also been a D cup or even a B cup if they used Sister Sizing to find a better fit.

You really do need to have an idea of what the band size was on your C cup to get a sense of the size of breast we’re referring to. 

I used to be a 30F cup back in my early twenties. That breast was significantly smaller than my 32G cup now. 

My projection back then was only 5-6 inches on a 30 inch band vs the 7-8 inches today on a 32 band. I’d bet that my boobs were each closer to the size of an asian pear back then and now are on par with more of a large mango. 

And as you can infer by my band size, my weight was significantly smaller back then as well :)

What this comes down to ultimately is finding the best fit. And what kind of band size you need with what kind of cup you fill. 

I personally own many different sizes of bras because I can fluctuate so much in any given month. But whenever I am feeling bloated, I ditch the 32Gs I normally wear and slide into a 34F.


So a more accurate gauge of my size would be to say I’m between a 32-34 F-G cup. It’s a mouthful, but a lot more realistic in any given month than just saying I’m a “G cup.”


Ultimately, my cup size is just one small thing about me as a person. It doesn't tell you that I'm a total bibliophile and love to read books. Or that I'm a hot mess in arranging a dishwasher efficiently no matter how much I try. It matters a lot for bra sizing, but says little else about me as a person.

Hopefully cup sizing makes a bit more sense now and I’d like to say that there’s a moment when bra sizing becomes completely intuitive. 

However, like everything else about me and this industry, it’s complicated :)