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What’s the Difference Between 30B, 32B, 34B, 36B, 38B, 40B, 42B, 44B, 46B and 48B bra sizes?

Bra sizes can be confusing. Many people think a 34B and a 38B are the same because of the “B cup.” But that’s not true. Bra sizes are made up of two parts: the band size (the number) and the cup size (the letter).

This guide will take you through what each B-cup size from 30B to 48B actually means. By the end, you’ll see how the sizes change, why they fit different body types and how you can use this knowledge to choose a bra that feels comfortable and supportive.

Understanding Bra Size Basics: Band + Cup

Each bra size includes two essential elements that work together to ensure a proper fit. One is the band size, which supports the bust from underneath and the other is the cup size, which reflects the volume of the breasts.

Band Size (30, 32, 34, etc.) - This is the measurement around your ribcage, just under your bust. The band gives most of the support in a bra. A smaller number means a smaller ribcage and a bigger number means a larger ribcage or wider frame.

Cup Size (B in this case) - The letter shows how much the bust projects out from the chest wall. A “B” cup is considered a moderate or average size, but it will not look the same across all bands.

Here’s the important point: a 30B and a 44B are both B cups, but the actual cup volume is different because the band sizes are different. The larger the band, the larger the cup volume becomes, even though the letter stays “B.”

Bra Size Comparison Chart

Here’s how each B-cup differs in fit and who it usually suits best:

Bra Size Band (inches) Cup Volume Best For
30B  25–26 Very small B cup Petite, narrow frames with a small bust
32B  27–28 Slightly fuller B cup Slim or average ribcage, small bust
34B  29–30 Moderate B cup Average builds needing balanced fit
36B  31–32 Fuller B than 34B Broader ribcage, moderate bust
38B  33–34 Larger B cup volume Wider frames, plus-size with smaller busts
40B  35–36 Wider band, moderate bust Curvier builds, plus-size women
42B  37–38 Fuller, deeper B cup Plus-size women needing stronger band support
44B  39–40 Broader B cup Fuller body frames, smaller busts
46B  41–42 Larger frame, smaller bust Plus-size women wanting secure fit
48B  43–44 Broadest B cup size Extended plus-size frames with small busts

So yes, all these bras are B cups, but just like the A-cup example, the cup volume increases as the band size increases. That’s why a 30B and a 48B don’t look the same, even though both are called “B.”

Exploring the Most Common B Sizes

To make things clearer, here’s a closer look at some of the more common B-cup sizes:

  • 34B Bra Size: This is often called a “standard” B cup. Fits women with an average ribcage and moderate bust volume. One of the most common sizes.
  • 36B Bra Size: It has a slightly bigger band and a deeper cup than a 34B. Best for women with a broader chest or wider ribcage.
  • 38B Bra Size: This offers more cup space and band support. Popular among plus-size women who have a smaller bust compared to their band size.
  • 40B Bra Size: It is designed for plus-size women with a moderate bust. The wider band provides better stability and comfort throughout the day.

Let’s Talk Sister Sizes

Sometimes, your regular bra size might not feel quite right — maybe the band is too tight or the cups don't sit properly. Or perhaps your exact size isn’t available in a particular brand or style. That’s where sister sizes come in handy.

Sister sizes are bra sizes that have the same cup volume, but the band size and cup letter differ. When you go up or down a band size, adjusting the cup size in the opposite direction helps maintain the same overall fit in the cup.

For example:
If you wear a 34B, your sister sizes are 32C and 36A.

  • If the 34B feels too tight around the band, try a 36A.
  • If it feels too loose, try a 32C.

This is a helpful trick when you're between sizes, experiencing body changes or shopping across different brands — since sizing can vary slightly from one label to another.

Understanding sister sizes can make bra shopping a lot easier and ensure you find a comfortable, supportive fit.

How to Find the Right B Cup Size

Here are some easy tips to get the right B size for you:

  1. Measure properly - Use a tape measure around your ribcage (underbust) to find your band size and across the fullest part of your bust to find your cup size.
  2. Don’t focus only on the label - Comfort is more important than just the numbers and letters.
  3. Try different styles - T-shirt bras give a smooth look, balconette bras provide lift and sports bras are great for active days.
  4. Test the fit - Move, stretch and sit while trying a bra. If it slips, digs or feels uncomfortable, it’s not the right one.
  5. Check different brands - Sizes may feel slightly different depending on the brand, so it’s worth trying more than one.

From 30B to 48B, all these sizes are B cups, but the fit changes with the band. The same letter “B” does not look the same across every band size. That’s why a 30B is much smaller than a 48B.

The best size for you is the one that feels comfortable, supports your body and makes you feel confident. Don’t be afraid to try sister sizes if one doesn’t feel right.

At Shyaway, we offer a wide range of B-cup bras across many band sizes, so you can find the one that truly suits your shape and style.

Everyday Styles That Suit B Cups

Not every bra style works the same for every band size, even if the cup letter is the same. Here are some everyday styles that usually suit B cups across different body types:

  • T-shirt Bras – Great for daily wear. They give a smooth finish under tops and dresses, especially for 32B–38B.
  • Balconette Bras – Perfect for outfits with wider or lower necklines. They lift gently without too much padding.
  • Wire-Free Bras – Comfortable and light, best for 30B–34B where support needs are lower.
  • Minimiser Bras – A smart choice for women in 38B–44B who want a neater shape under fitted clothes.
  • Sports Bras – Essential for every size, from 30B to 48B. Look for wide bands in larger sizes for better support.
  • Full-Coverage Bras – Ideal for plus-size B cups (40B–48B). They prevent spillage and give all-day comfort.

The key is to match the style with your body frame and outfit. While a 30B may enjoy bralettes and light padding, a 46B might prefer wider bands and full coverage for balance.

Every woman’s body is different, and so is every B cup. The journey from 30B to 48B shows us that bra sizes aren’t one-size-fits-all—they’re meant to adapt to different frames, shapes and comfort needs.

The right size is the one that makes you feel supported without effort, confident without trying, and comfortable all day long. Whether you’re petite or plus-size, there’s a perfect B cup out there for you—waiting to make both your outfits and your confidence shine.

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