If you've been led to believe A cups are "small" boobs and D cups are "big" boobs (and therefore DD+ are outrageously big) this is for you! It's time everyone understands what that little tag on bras really means! And most importantly, what it means for you.
When shopping for a bra, terms like bra size, cup size, and breast volume often come up, but many people find them confusing (because they can be!). Understanding how these elements relate can help you find a perfectly fitting bra, boost your confidence, and ensure comfort.
What is Bra Size?
Bra size is a combination of two measurements: the band size and the cup size. It’s typically expressed as a number followed by a letter, such as 34B or 38D.
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Band size refers to the measurement around your ribcage, just under your bust, in inches (in the U.S.) or centimeters (in many other countries).
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Cup size indicates the volume of your breasts relative to your band size, represented by letters like A, B, C, D, and beyond. This is where most of the confusion arises, but cup size alone means nothing. It's a relative measurement!
Together, these components form your bra size, but they don’t tell the whole story about breast volume. Let’s dive deeper.
What is Cup Size?
Cup size is often misunderstood as a fixed measurement of breast size, but it’s actually relative to the band size. This is why a 32C and a 36C have different breast volumes, even though they share the same cup letter.
Cup size is determined by the difference between your bust measurement (around the fullest part of your breasts) and your band measurement:
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A 1-inch difference = A cup
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A 2-inch difference = B cup
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A 3-inch difference = C cup
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A 4-inch difference = D cup
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And so on, with each additional inch increasing the cup size.
For example, a woman with a 34-inch band and a 37-inch bust has a 3-inch difference, making her a 34C. However, cup sizes scale with band sizes, which leads us to breast volume.
What is Breast Volume?
Breast volume refers to the actual three-dimensional space your breasts occupy, typically measured in cubic centimeters (cc). Unlike bra or cup size, breast volume is a more precise way to quantify breast size, often used in medical contexts like breast augmentation or reduction surgeries.
Here’s the key point: Cup size does not directly equal breast volume. Two women with the same cup size but different band sizes will have different breast volumes. For instance:
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A 32D bra holds less breast volume than a 36D bra, even though both are “D cups.”
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This is because cup size is proportional to band size. A larger band size requires a larger cup to maintain the same proportional fit.
To illustrate, studies estimate that a D cup on a 32 band might hold around 400–500 cc of breast volume, while a D cup on a 38 band could hold closer to 700–800 cc. These numbers vary depending on the bra brand and breast shape.
The Sister Sizing Concept
You can read lots more about sister sizing in this blog post.
Bras have “sister sizes,” which are different bra sizes that hold roughly the same breast volume. Sister sizing works by adjusting the band and cup size inversely:
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Going up one band size and down one cup size (e.g., 34C to 36B) maintains similar cup volume.
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Going down one band size and up one cup size (e.g., 34C to 32D) also keeps the same volume.
For example, a 32D, 34C, and 36B are sister sizes with comparable breast volumes, even though they look different on paper. This is why trying on sister sizes can help you find a better fit if a bra feels too tight or loose in the band.
Why Breast Volume Matters
Understanding breast volume is crucial for several reasons:
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Better Bra Fit: Knowing that cup size isn’t a universal measure of breast size helps you choose bras that match your actual breast volume. This is especially important for women with larger band sizes or unique breast shapes.
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Surgical Decisions: In breast augmentation or reduction, surgeons focus on volume (in cc) rather than cup size. A patient requesting a “C cup” might need different implant sizes depending on their band size and existing breast tissue.
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Body Positivity: Recognizing that bra sizes are relative can reduce frustration when comparing your size to others. A 36A and a 32C might have similar breast volumes, despite societal perceptions of cup letters.
Factors Affecting Bra Fit Beyond Volume
While breast volume is important, other factors influence how a bra fits:
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Breast Shape: Breasts can be shallow, projected, wide-set, or close-set, affecting how they fill a bra cup. We posted two blog articles recommending bras for shallow breasts and for projected breasts.
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Bra Style: Different styles (e.g., balconette, plunge, or full-coverage) fit differently, even in the same size.
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Brand Variations: Bra sizing isn’t standardized across brands. A 34C in one brand might fit like a 32D in another.
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Body Changes: Weight fluctuations, pregnancy, or hormonal shifts can alter breast volume and bra size over time.
How to Find Your Correct Bra Size
To find a bra that fits your breast volume and body, follow these steps:
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Measure Your Band Size: Use a soft tape measure to measure around your ribcage, just under your bust. Round to the nearest even number.
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Measure Your Bust Size: Measure around the fullest part of your breasts, keeping the tape level and snug but not tight.
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Calculate Cup Size: Subtract your band measurement from your bust measurement. Use the difference to determine your cup size (see the chart above).
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Try Sister Sizes: If your calculated size feels off, try sister sizes to find the best fit.
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Visit a Professional: Many lingerie stores offer free fittings to help you find your perfect size.
Final Thoughts
The relationship between bra size, cup size, and breast volume is complex but empowering to understand. Bra size combines band and cup measurements, while cup size is relative to the band, not a direct measure of breast volume. Breast volume, measured in cubic centimeters, varies across band and cup sizes, even for the same cup letter. By grasping these concepts and exploring sister sizes, you are well on your way to finding a bra that fits comfortably and flatters your own shape.
If you have any questions about bra measuring, fitting or would like us to stock a size in a specific brand or style — send us a message!
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