You're probably wearing the wrong bra. Most people are. Honestly, the industry has spent decades convincing us that we’re all a 34B or a 36C because those are the sizes they like to mass-produce. It’s annoying. If your straps are digging in, your wires are poking your armpits, or you’re "quad-boobing" over the top of the lace, your math is off. Learning how to find out my cup size isn't actually about the letter on the tag; it's about the ratio between two specific measurements.
It sounds scientific. It's not.
Standard sizing is a mess because every brand—from Victoria’s Secret to PrimaDonna—uses different "grading" scales. A "D" cup in France is not the same as a "D" cup in the US or the UK. This is why you feel like a crazy person in the fitting room. To get it right, you need a flexible measuring tape, a mirror, and zero ego about the numbers. Forget what you think you are. Let's look at the actual physics of your body.
The Tape Measure Doesn't Lie (Usually)
Grab a soft measuring tape. If you don't have one, use a piece of string and then hold it up to a ruler later. Stand in front of a mirror. You need to be naked or in a very thin, non-padded bra. If you wear a push-up while measuring, you’re just measuring the foam, which defeats the whole purpose.
First, wrap the tape around your ribcage, directly under your bust. This is your band size. Keep the tape snug. I mean really snug. It should be level all the way around—if it’s dipping at the back, your measurement will be too big.
Exhale.
Read the number. If you get an odd number like 33 or 35, you’ll usually want to try both the size down and the size up in a store, but for the sake of the "math," we’ll round. Some old-school fitters tell you to add four inches to this number. Don't do that. That’s a relic from the 1940s when fabrics weren’t stretchy. If your ribs measure 30 inches, your band size is likely a 30 or a 32. Adding four inches would put you in a 34, which will slide up your back and provide zero support.
Now for the bust. Wrap the tape around the fullest part of your chest. Don't pull it tight this time. Just let it rest. This is where people get confused about how to find out my cup size because breast tissue is soft and moves. If you have "shallow" tissue or your breasts sit lower, you might actually want to lean forward at a 90-degree angle so the tissue hangs down. This captures the full volume that needs to fit inside a cup.
Doing the Mental Math
The cup size is simply the difference between the two numbers. Each inch represents a letter.
- 1-inch difference = A
- 2-inch difference = B
- 3-inch difference = C
- 4-inch difference = D
- 5-inch difference = DD (or E in some UK brands)
If your ribs are 32 and your bust is 35, you're a 32C. If your bust is 38, you’re looking at a 32DDD or F. It sounds huge, right? It isn't. A 32F is actually a very common size, but because most department stores stop at D, they shove everyone into a 36D instead. This is called "sister sizing," and while it works in a pinch, it’s why your bra feels like a torture device by 4:00 PM.
Why Your Current Bra Is Lying To You
There’s a concept in the bra world called "The Orange in a Glass" effect. Imagine trying to put an orange into a narrow glass. The orange won't go in, even if the glass is technically "big" enough to hold the volume. This happens when your underwire is too narrow. The wire sits on top of your breast tissue instead of behind it. This makes the cup look "empty" at the top, leading you to think, "Oh, I need a smaller cup."
Nope. You actually need a larger cup with a wider wire.
Expert fitters like those at Rigby & Peller or the famous Bravissimo in the UK often ignore the tape measure entirely after the first minute. They look at the "tack." The "gore"—that little triangle of fabric between the cups—should sit flat against your sternum. If it’s floating, your cups are too small. Your breasts are pushing the whole bra away from your body.
Then there’s the "scoop and swoop." This is non-negotiable. When you put a bra on, you have to literally reach into the cup, grab the tissue from under your armpit, and pull it forward into the cup. Most of what we think is "armpit fat" is actually just breast tissue that has been squished out of the cup for years by ill-fitting bras.
The International Confusion
If you’re shopping online, God help you. US sizing goes D, DD, DDD, G, H. UK sizing (which is much more consistent) goes D, DD, E, F, FF, G.
If you find a brand you love like Panache or Freya, check the tag. They usually list multiple countries. Always go by the UK size if you can find it; it’s the gold standard for consistency in the industry. European brands like Chantelle use centimeters, which is a whole different headache. Basically, a 70 band in Europe is a 32 band in the US.
Signs You've Nailed the Size
You'll know you've figured out how to find out my cup size correctly when the following things happen:
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- The band stays parallel to the floor and doesn't ride up toward your shoulder blades.
- You can fit two fingers under the band, but not more.
- The underwire follows the natural crease of your breast and doesn't poke your breast tissue.
- The straps stay up without being cinched so tight they leave red welts. (Remember: 80% of the support comes from the band, not the straps!)
Troubleshooting Common Issues
Maybe you measured and the math says you're a 34D, but the 34D you just bought has gaps at the top. This doesn't necessarily mean you're a C. It might mean you have a "full on bottom" shape. You need "half-cup" or "balconette" styles rather than "full coverage" or "plunge" styles.
Shape is just as important as volume. Think of it like shoes. You can be a size 8, but if you have a wide foot, a pointy size 8 stiletto will be agonizing while a size 8 sneaker is fine.
If you have one breast significantly larger than the other (which is totally normal, by the way), always fit the larger one. You can always add a small insert (a "chicken cutlet") to the smaller side, but you can't hide a "double-boob" bulge on the larger side.
Your Action Plan for a Better Fit
Stop relying on the 10-year-old bra in your drawer. Elastic dies. If your bra is more than six months old and you wear it twice a week, the band has likely stretched out by at least an inch, meaning your "size" in that specific bra is now wrong.
- Step 1: Get a fresh measurement today using the "tight ribcage" and "leaning forward bust" method.
- Step 2: Identify your "starting point" size based on the inch-difference math.
- Step 3: Go to a store with a wide range of brands. Try on one bra in your calculated size, one cup size up, and one band size down.
- Step 4: Perform the "scoop and swoop" every single time you hook the back.
- Step 5: Check the gore. If it doesn't touch your skin, go up a cup size.
Once you find a brand and size that works, stick with it, but remember to re-measure if your weight fluctuates by more than 5 or 10 pounds. Your chest is often the first place to change. Finding your real size is a bit of a journey, but your back and shoulders will thank you for the effort.