Cup sizes are perhaps the most misunderstood aspect of bra sizing. Many women believe cup sizes are absolute measurements—that a D cup is always "large" and an A cup is always "small." In reality, cup sizes are relative measurements that change meaning depending on the band size they're paired with. Understanding this fundamental concept transforms how you approach bra shopping.
The confusion around cup sizes contributes significantly to the widespread problem of women wearing the wrong bra size. Once you understand what cup sizes actually represent, you'll be better equipped to find bras that fit perfectly and feel comfortable all day long.
What Does Cup Size Actually Measure?
A cup size represents the difference between your bust measurement and your underbust measurement—nothing more and nothing less. It's a measurement of volume relative to your band size, not an absolute measurement of breast size.
In the Australian and UK sizing systems, each inch (or approximately 2.5cm) of difference between bust and underbust corresponds to one cup size. If your bust measures 4 inches larger than your underbust, you're a D cup. If it measures 6 inches larger, you're an F cup. Our Australian bra size calculator does this calculation for you automatically.
This relative measurement means that cup size must always be considered alongside band size. A D cup on a size 10 band contains less actual volume than a D cup on a size 16 band, even though they share the same cup letter.
The Band Size Connection
Here's the key insight that changes everything: cup volume scales with band size. As band size increases, the same cup letter represents a larger actual cup. As band size decreases, the same cup letter represents a smaller actual cup.
Think of it this way: a 10D and a 16B have similar cup volumes, even though their cup letters are quite different. Both hold approximately the same amount of breast tissue—the difference is in the band length and how that volume is distributed.
This relationship is the foundation of sister sizing, where you can move between sizes that share the same cup volume but have different bands. Understanding this connection helps explain why two women can wear the same cup letter but appear to have very different breast sizes.
Why Many Women Wear the Wrong Cup Size
The most common fitting mistake is wearing a cup size that's too small. This happens for several reasons, many of them rooted in cup size misconceptions.
First, many women underestimate their cup size because of lingering cultural associations between larger cup sizes and being "busty." This leads them to squeeze into smaller cups rather than trying the larger size that would actually fit correctly.
Second, the relationship between band and cup size means that going up in band size without adjusting cup size actually gives you a smaller cup volume. A woman might start with a correctly fitting 12D, gain weight, and move to a 14D—but the 14D cup, while sharing the same letter, is actually one cup size smaller in volume relative to her body. She should instead try a 14DD to maintain the same cup volume.
Third, retail environments often stock limited size ranges, pushing women toward available sizes rather than correct ones. Many stores carry primarily 10-14 bands in B-D cups, creating the impression that these are "normal" sizes when in fact a much broader range exists and is commonly needed.
Cup Size Progression Explained
In Australian and UK sizing, cup sizes follow a specific progression: AA, A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K, KK, and beyond. The doubled letters (DD, FF, GG, HH, JJ, KK) are full cup sizes, not half sizes—there's as much difference between D and DD as between B and C.
American sizing differs after D cup, typically using DD, DDD (or F), DDDD (or G), with less standardisation in the larger sizes. European sizing also uses its own progression, which varies by country. When shopping internationally, always check conversion charts carefully.
Each step in the cup progression represents approximately 2-2.5cm (about 1 inch) of additional bust-to-underbust difference. This standardised progression makes it possible to predict how changing cup sizes will affect fit.
Finding Your True Cup Size
The only reliable way to determine your cup size is through accurate measurement. You need two measurements: underbust (around your ribcage directly under your breasts) and bust (around the fullest part of your chest, typically at nipple level).
The difference between these measurements indicates your cup size. Our bra size calculator performs this calculation using Australian sizing standards, giving you your cup size along with your complete bra size and international conversions.
It's worth noting that cup size can vary slightly depending on breast shape and how you take measurements. Two women with identical measurements might find that different cup sizes work better for them depending on whether they have shallow versus projected breasts, or whether their fullness is distributed more toward the top or bottom of the breast.
Common Cup Size Myths Debunked
Myth: D cups are large. Reality: D cups are often portrayed as large in popular culture, but a D cup represents only about 4 inches of difference between bust and underbust. Many women who think they're B or C cups are actually D cups or larger. The perceived "largeness" of D cups is often actually larger sizes (DD, E, F, and beyond) that have been incorrectly identified.
Myth: Cup sizes above DD are rare. Reality: Cup sizes above DD are extremely common—just underrepresented in many retail environments. Bra companies that specialise in extended sizes report that their most popular sizes often fall in the DD-G range.
Myth: Your cup size stays the same across different band sizes. Reality: If you change band sizes, you typically need to adjust your cup size to maintain the same cup volume. Going up a band size and down a cup letter (or vice versa) keeps approximately the same cup volume.
Myth: Small-banded women can't have large cup sizes. Reality: Band size and cup size are independent measurements. A woman with a small ribcage can absolutely have significant bust-to-underbust difference. Sizes like 10F, 8FF, and 10G exist precisely because small-banded women with larger cup sizes are completely normal.
Why This Understanding Matters
Understanding how cup sizes actually work liberates you from arbitrary limitations when shopping for bras. You'll no longer dismiss sizes because the letter seems "too high" or the band seems "too small." Instead, you'll evaluate bras based on whether they fit your body correctly.
This knowledge also helps you communicate effectively with sales assistants and make sense of size charts. When you understand that a DD isn't dramatically larger than a D—it's just one inch more difference—you're less likely to be intimidated by cup letters and more likely to try the size that actually fits.
Use our Australian bra size calculator to discover your true cup size based on your actual measurements. You might be surprised by the result, but trust the mathematics—and trust how the correct size feels when you try it on.