Most people think a C-cup is "average" and a DD is "huge." They're wrong. Honestly, the way we talk about all boob cup sizes is fundamentally broken because we treat the letter like a fixed volume. It isn't. A 32D and a 40D are not the same size. Not even close. If you put the underwire from a 32D against a 40D, the 40D wire would look like a salad plate while the 32D looks like a coaster.
Bra sizing is a math problem that the fashion industry has tried to turn into a marketing gimmick. We’ve been fed this idea that A means small, B means medium, C means large, and D means "extra large." That's total nonsense. In reality, the cup size is just the relationship between two numbers: your ribcage and your bust. That’s it.
The Sister Size Secret
Ever wonder why you can sometimes fit into a 34C and a 36B? It’s not magic. It's sister sizing. This is probably the most important thing to understand when looking at all boob cup sizes. If you go up in the band, you have to go down in the cup to keep the same internal volume.
A 30D has the exact same amount of "boob volume" as a 32C, a 34B, and a 36A. They are identical in capacity. If you don't believe me, ask a professional fitter at a place like Nordstrom or a specialized boutique like Rigby & Peller. They see this every day. Women walk in wearing a 36C because they think they’re "average," but they actually belong in a 32DDD. The industry calls this "the plus-four method," where brands tell you to add four inches to your ribcage measurement just so they can squeeze you into the limited range of sizes they mass-produce. It’s a lie designed to save manufacturing costs.
How the alphabet actually works
The math is actually pretty simple, even if the results feel chaotic.
- Measure your underbust (snug around the ribs).
- Measure the fullest part of your bust.
- Subtract the first from the second.
Each inch of difference represents one letter. One inch is an A, two inches is a B, three is a C, and so on. But here is where it gets weird. In the UK, the sizing goes D, DD, E, F, FF. In the US, it often goes D, DD (or E), DDD (or F), G, H. There is no global standard. A "G cup" from a French brand like Simone Perele is going to fit completely differently than a "G cup" from a British brand like Panache.
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Beyond the Letter: Shape Matters More Than Size
You could have two people with the exact same measurements—say, a 34DD—and the same bra will fit one perfectly and look terrible on the other. Why? Shape.
Breasts aren't just volumes; they have "roots" and "projection." Some people have "shallow" breasts where the tissue is spread out over a large area of the chest. They might measure as a 30E but look like what society calls a "B cup" because the tissue isn't concentrated in one place. Then you have "projected" breasts that stick out further from the chest wall.
Then there’s the "root" issue. If your breast tissue starts high up near your collarbone, you have "tall roots." If you try to wear a balconette bra, it might cut into the top of your tissue, creating that "quadra-boob" look, even if the cup size is technically correct. Conversely, "short roots" might leave gaps at the top of the cup, making you think the bra is too big when it’s actually just the wrong shape.
The D-Cup Delusion
We need to talk about the "double D." For some reason, pop culture has decided that DD is the finish line for breast size. In reality, DD is still on the smaller-to-medium side of the spectrum when paired with a small band size. According to data from the "Bra Fitting" subreddit (a community of over 300,000 members dedicated to sizing accuracy), the most common size for someone who thinks they are a 34B is actually closer to a 30F.
The "bra matrix"—that 32A to 38DD range you see at Victoria’s Secret—covers maybe 30% of the actual population. Most people are walking around in bands that are too big and cups that are too small. This causes the "boob hat" effect. That's when the bra just sits on top of the breasts instead of encircling them. If your underwire is sitting on breast tissue at the sides, your cup is too small. Period.
Why Your Size Changes Every Month
Fluctuation is the only constant. Hormones, weight changes, and even hydration levels affect breast volume. Research in the Journal of Female Health Sciences has noted that breast volume can vary significantly throughout the menstrual cycle due to water retention and blood flow.
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If you're buying a bra, don't buy it for your smallest day. Buy it for your "average" day. And always buy a bra that fits on the loosest hook. Why? Because bras are made of elastic. As you wear and wash it, the band will stretch. If you start on the loosest hook, you can move to the tighter ones over time to keep the support. If you start on the tightest hook, you’ve got nowhere to go once the bra starts to age.
The Weight Factor
It's a common myth that losing 10 pounds means you'll drop a cup size. Not necessarily. Breasts are a mix of fatty tissue and glandular tissue. If your breasts are mostly glandular, they won't change much even if you lose weight elsewhere. If they are mostly fat, they’ll be the first thing to go. This is why some "plus-size" individuals have smaller cup sizes than "straight-size" individuals. It’s all down to genetics and tissue density.
Actionable Steps for Finding Your Real Size
Stop guessing. Seriously. If you want to actually understand all boob cup sizes as they apply to your own body, you need a tape measure and ten minutes.
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- Use the "A Bra That Fits" calculator. It's the gold standard. It uses six different measurements (leaning, lying down, standing, etc.) to account for tissue projection.
- Check the "Scoop and Swoop." This is non-negotiable. When you put on a bra, reach into the cup and pull all the tissue from the sides and underarms toward the center. Most people find that they "overflow" the cup once they do this correctly, proving their current cup is too small.
- Look at the Gore. The little piece of fabric between the cups should sit flat against your sternum. If it's floating or hovering, your cups are too small.
- Ignore the label, trust the fit. If you end up being a 28GG, don't panic. It doesn't mean you're "huge." It just means your ribs are small and your tissue needs a certain amount of room.
- Shop UK brands. If you are above a DD, brands like Freya, Panache, and Elomi offer much more consistent sizing and better engineering than most US "mall brands."
The reality of breast sizing is that it's highly individual. A "size" is just a starting point for a conversation between your body and a piece of clothing. Once you stop fearing the letters further down the alphabet, you'll find that back pain disappears and your clothes actually fit the way they were designed to.
Stop trying to fit your body into a "standard" size. The standards were made by people trying to sell you five sizes of the same cheap bra. You deserve the support that actually matches your anatomy.
Next Steps:
- Grab a soft measuring tape and record your underbust (snug) and full bust (leaning forward at 90 degrees).
- Subtract the numbers to find your starting cup letter.
- Test your current bra using the "scoop and swoop" method to see if you are actually filling the cups or if they are just "sitting" on you.