You’ve been there. You are standing in a dimly lit dressing room, surrounded by a mountain of lace and underwire, and nothing fits. The 34C is pinching your ribs, but the 36C feels like a loose tent. You’re annoyed. You might even be a little bit sweaty. This is usually when a sales associate pokes their head through the curtain and mentions bra size sister sizes.
It sounds like some weird, secret sorority math. Honestly, most people just assume if a 34C is too tight, they should just go up to a 36C. But that’s exactly where everything goes wrong. If you increase the band size but keep the cup letter the same, you’re actually getting a larger cup, too. It’s a total mess.
The reality is that bra sizing isn't linear. It’s a ratio.
The Math Behind Bra Size Sister Sizes
Basically, the volume of a "C cup" isn't a fixed amount of space. A 32C cup is significantly smaller in volume than a 38C cup. Think of it like a scoop of gelato. A small scoop on a tiny cone looks huge; that same scoop on a giant waffle cone looks like a rounding error. When you change your band size, you have to adjust your cup letter in the opposite direction to keep the same internal volume for your actual breast tissue.
This is the core of bra size sister sizes. If you go up in the band, you go down in the cup. If you go down in the band, you go up in the cup.
Let's look at a 34C. If the band is digging into your skin like a tourniquet but the cups feel perfect, you need a larger band. If you just grab a 36C, the cups will suddenly be too big because a 36C is built for more volume than a 34C. To keep that same "C-sized" cup volume on a larger 36 band, you have to drop to a 36B.
That 36B is the sister size.
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It feels counterintuitive. Our brains are wired to think "C is bigger than B." But in the context of a 36 band, that B cup holds the exact same amount of "stuff" as the 34C cup. Brands like ThirdLove and Wacoal have been screaming this from the rooftops for years, yet the "standard" shopping experience still leaves people guessing.
Why Does This Actually Work?
It’s all about the bridge.
The center part of the bra—the gore—needs to lay flat against your sternum. If your band is too loose, the back of the bra will hike up toward your shoulder blades. When the back goes up, the front goes down, and suddenly your support is gone. Gravity wins. By shifting to a sister size, you can find a band that actually stays parallel to the floor without losing the cup fit you already liked.
Take a 32DD. If that band feels like it’s suffocating you, your sister size is a 34D. Notice how the cup letter went down as the number went up? That's the magic trick. You’re maintaining the volume while increasing the circumference.
Common Sister Size Paths
- If you are a 30D: Your larger band sister size is a 32C. If you need an even bigger band, it’s a 34B.
- If you are a 36DD: Your smaller band sister size is a 34DDD (or 34E in some UK brands). Your larger band sister size is a 38D.
- If you are a 38B: Going tighter? Try a 36C. Going looser? Try a 40A.
It’s not perfect. It’s a starting point. Different brands use different "molds." A 34C from Victoria's Secret is going to feel world's apart from a 34C from a brand like Panache or Freya, which use UK sizing. UK brands often have more consistent grading in larger cup sizes, whereas US brands tend to get a bit wonky once you pass a D cup.
The Problem with "Standard" Retail Sizing
Most department stores only carry a narrow range. They love the "32 to 38, A to DD" sweet spot. It’s profitable. But the average woman doesn't always fit into that narrow window.
If you walk into a store and you’re actually a 30F, a salesperson might try to "sister size" you into a 34D just because that’s what they have in stock. Don't do it. There is a limit to how far you can jump. Generally, you can move one sister size away from your true measurement and still get decent support. Once you move two or three steps away, the proportions of the bra change too much. The straps will be too wide for your shoulders, or the underwire will start poking you in the armpit.
The underwire is the giveaway. In bra size sister sizes, the wire width changes. A 36B wire is wider than a 34C wire, even if the cup volume is the same. If you have narrow roots (meaning your breasts don't wrap far around toward your sides), a wider sister size might feel like the wire is stabbing your ribcage.
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Real World Testing: It’s Not Just About the Tape
Measurements lie.
You can use the "Add 4" rule (which is outdated and mostly used by brands to fit more people into fewer sizes), or you can use the "A Bra That Fits" method which uses six different measurements. Even then, your tissue density matters. If you have "soft" tissue, you might need to size down in the cup to avoid gaping. If you have "firm" tissue, you might need to go up.
I’ve seen people who swear they are a 36B for ten years, only to realize they are actually a 32E. They were using a huge band to compensate for cups that were way too small. When the cup is too small, your breasts push the bra away from your body, making the band feel tight. You think, "I need a bigger band," when you actually just need a bigger cup.
The sister size trick is a diagnostic tool. If you put on a bra and the cups fit but the band is loose, go down a band and up a cup letter. If the cups fit but the band is tight, go up a band and down a cup letter.
The Actionable Truth for Your Next Fitting
Stop looking at the letter as an ego thing. A "Double D" isn't actually huge. It just means there is a five-inch difference between your ribcage and the fullest part of your chest. In a 30 band, a 30DD is quite small. In a 42 band, a 42DD is quite large.
Here is how you actually use this info next time you shop:
- Check the Band First: Put the bra on backwards and upside down so the cups are hanging down your back. This tells you if the band fits without the cups interfering. If it stays up and feels snug, that’s your band size.
- The Two-Finger Rule: You should only be able to pull the band about two inches away from your spine. Any more, and it's too big.
- Identify the Issue: If the band is perfect but you’re spilling out the top (the "quadra-boob" effect), you need a larger cup in the same band. If the band is perfect but the cups are wrinkling, you need a smaller cup.
- The Sister Shift: If you find a cup that holds you perfectly but the band is physically painful, that is the only time you should use bra size sister sizes. Move up one band number and down one cup letter.
Don't be afraid of the "weird" letters like F, G, or H. US brands are notorious for making up their own naming conventions after D, so always check if a brand uses UK or US sizing. If you see "FF," you're looking at UK sizing. If you see "DDD," it's likely US.
The "right" size is whichever one stays parallel, supports from the bottom, and doesn't require you to adjust the straps every twenty minutes. Sometimes that means ignoring the number you've worn since college and trusting the sister size math instead. Your shoulders will thank you. Your posture will probably improve, too. And honestly, you’ll just look better in your clothes when everything is sitting where it’s actually supposed to be.