What's the real difference between an A cup and a B cup? Or a D and a DD? This cup size comparison shows what each size actually means -- in inches, volume, and everyday terms. Whether you're comparing A cup vs B cup, B cup vs C cup, or C cup vs D cup, this guide covers every step.
Cup size comparison is the process of understanding how bra cup letters relate to each other in terms of volume, projection, and fit across different band sizes. A cup letter represents the inch difference between bust and underbust measurements -- A equals 1 inch, B equals 2 inches, C equals 3 inches, and D equals 4 inches -- but the actual cup volume changes depending on the band size. A 32D holds approximately 325 ml while a 38D holds roughly 430 ml, even though both carry the same letter. This relationship exists because cup molds are scaled proportionally to the band circumference, a principle defined in the ASTM D6240 standard for women's bra sizing. According to a 2024 survey of 1,075 US women by Bratabase, 61.9% wear sizes outside the industry's core range, suggesting that most women have not accurately compared their measurements to the sizing chart. A 32D and a 38D look different because the D refers to the same 4-inch difference applied to a smaller versus larger frame. For sizes that share the same cup volume across bands, see sister sizes.
Want to know your actual size before comparing? Try our bra size calculator first.
Our visual cup size comparison below shows approximate volume at band 34. Hover for details.
Volumes are approximate and vary by band size. A D cup at band 32 is smaller than a D cup at band 38.
This cup size comparison continues with real visuals. These four women all wear the same band size -- the only variable is cup volume.
From A cup (1-inch difference) to DD cup (5-inch difference). Each letter adds approximately one inch of bust projection.
Less than one inch of difference between bust and underbust. The smallest standard cup size. If you're looking for an AA cup size bra, specialty brands and bralettes are your best options. Common in teens and petite frames. Approximate volume at band 34: around 65 ml. Bralettes and unlined styles tend to fit best.
One inch of difference. A subtle curve that's well-suited to triangle bras and lighter support. Volume at band 34: roughly 130 ml. Many women sized as an A in stores actually measure as a B or C with proper fitting -- the +4 method systematically undersizes cups.
Two inches of difference. A moderate cup that works across most bra styles. Volume at band 34: about 200 ml. Often considered "average" in the US, though actual average sizes trend larger. The jump from A to B is noticeable in fit but subtle visually.
Three inches of difference. Full enough to benefit from underwire support in most styles. Volume at band 34: approximately 275 ml. One of the most commonly stocked sizes, widely available across brands.
Four inches of difference. Despite its reputation, a D cup isn't necessarily "large" -- on a 30 band, it's quite compact. Volume at band 34: around 360 ml. Good support and well-fitted straps start to matter here.
Five inches of difference. The most commonly purchased size in the US is 34DD. In UK sizing, DD is the same. After this point, US and UK cup letters diverge. Volume at band 34: approximately 450 ml.
Six inches of difference. This is where US and UK sizing splits -- the US calls it DDD or F, the UK calls it E. Volume at band 34: about 550 ml. Full-coverage and minimizer styles become popular. Understanding the DDD cup size is key -- it's the same as an F cup in US sizing.
Seven inches of difference. Specialty brands like Elomi, Panache, and Freya become the go-to options, as most mass-market retailers stop at DD or DDD. Volume at band 34: roughly 660 ml. Proper fit becomes crucial for comfort.
Eight or more inches of difference. Sizes continue through I, J, K, and beyond in both US and UK systems (though the letters don't match). Volume at band 34 for an H cup: around 780 ml. Specialty retailers and online shopping become essential.
The most widespread misconception about bra sizing is that a cup letter represents a fixed breast size, when in fact it describes a proportional relationship between two body measurements. A D cup means a 4-inch difference between bust and underbust, regardless of body frame. On a 30-inch band, a D cup holds approximately 295 ml, while on a 40-inch band that same letter represents roughly 460 ml -- a difference of over 50%. This misunderstanding contributes to the statistic that an estimated 80% of women wear the wrong bra size, according to fitting data cited across the lingerie industry and discussed extensively in the r/ABraThatFits community wiki. Many women avoid D and DD cups because they assume the letters indicate large breasts, when in practice a 30D is a relatively small-framed size. The solution is measurement-based sizing rather than letter-based guessing, which is why using a bra size calculator with your actual measurements produces dramatically more accurate results than self-selecting a cup by visual assumption alone.
Need to find your measurements first? See our measuring guide for step-by-step instructions.
After reviewing this cup size comparison, check the complete size reference across all international systems in our bra size chart. To find sizes with the same cup volume on a different band, try our sister size finder. Cup volume is only one factor in bra fit -- breast shape determines which bra styles work best for each individual. For active wear, our sports bra vs regular bra comparison explains how cup support differs between everyday and exercise bras. Parents shopping for teens can use our teen bra guide to understand how cup sizes develop and which starter styles work best.
