Let's be real for a second. Most of us are walking around in bras that don't actually fit, and it's mostly because the industry has spent decades teaching us a math equation that is, frankly, kind of a disaster. You’ve probably heard it a thousand times: measure your ribs, add four or five inches, then subtract that from your bust measurement.
It's called the "Plus Four" method. It’s also wrong.
If you want to understand your cup size in inches, you have to throw away the idea that a "C cup" is a fixed volume. A 32C and a 38C are not the same size. Not even close. One is roughly the volume of a large orange; the other is more like a small grapefruit. When we talk about these measurements, we are actually talking about a ratio—the mathematical difference between the circumference of your ribcage and the fullest part of your chest.
The Math of the Difference
The logic is simple on paper. Each inch of difference represents a letter. One inch is an A, two inches is a B, three is a C, and so on. But here’s where it gets messy. Most people measure their underbust at, say, 31 inches and think, "Okay, I'm a 36 band." No. You are a 32 band.
When you add those arbitrary inches to your band size, you're shrinking the mathematical gap between your ribs and your bust. This leads to the "boob hat" phenomenon, where the bra just sits on top of your chest instead of actually supporting it. If your underbust is 30 inches and your bust is 34 inches, that 4-inch gap makes you a D cup. If you used the old-school "Plus Four" method, you’d end up wearing a 34AA. That's a massive difference in how a garment actually feels on your skin.
Honestly, the tape measure is just a starting point. Your breast tissue density matters just as much as the numbers. Some people have "shallow" shapes where the tissue is spread across a wide area, making them look smaller than their cup size in inches would suggest. Others have "projected" shapes where the tissue sits forward. Two people can have the exact same measurements and require completely different bra styles.
Why the Industry Standard is Broken
The "Plus Four" rule wasn't created for your comfort. It was created in the 1930s when bra materials didn't have much stretch. Manufacturers needed a way to make sure people didn't rip through the fabric, so they told women to size up. Even though we now have high-tech elastics and Lycra, many brands—especially big-box retailers—still use this outdated system because it allows them to fit more people into a smaller range of sizes.
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It’s cheaper to manufacture.
Think about it. If a store only carries 32 to 38 bands and A through DD cups, they have a limited inventory to manage. By telling a woman with a 30-inch underbust that she’s actually a 34, they can sell her a bra they already have in stock. This is why so many people struggle with straps falling down or bands riding up their backs. The band is the primary source of support, and if the math is wrong, the support is non-existent.
The Conversion Reality
If you're looking at your cup size in inches, here is how the standard US/UK progression usually looks:
- 0 inches: AA
- 1 inch: A
- 2 inches: B
- 3 inches: C
- 4 inches: D
- 5 inches: DD (E in UK sizing)
- 6 inches: DDD (F in UK sizing)
- 7 inches: G
Wait. It gets more confusing.
Once you get past a D cup, US and UK sizing diverge like a bad breakup. A US "G" is often a UK "F." If you are buying a bra from a European brand like PrimaDonna or a UK brand like Panache, you have to be incredibly careful. A "G cup" in London is much larger than a "G cup" in New York. Always look at the tag for the country of origin.
Shape Matters More Than the Tape
You could spend all day obsessing over whether you're a 3.5-inch difference or a 4-inch difference. But the "orange in a glass" analogy from the bra-fitting community is a classic for a reason. If you have a wide-set chest, you might need a wider wire even if your cup size in inches is technically smaller. If you have "omega" shaped breasts, the measurement around the fullest part might overstate your actual volume.
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Soft tissue also plays a role. If you’ve lost weight or finished breastfeeding, your tissue might be less firm. In this case, the tape measure might tell you you're a 34DD, but the lack of "fullness" at the top of the breast means you'll gap in a molded T-shirt bra. You might actually need a 34D or a different cup height altogether.
The Physical Toll of Getting it Wrong
This isn't just about fashion. It's about health.
When the band is too loose—because you added those "magic" four inches—the weight of your chest hangs entirely on the shoulder straps. This leads to those deep grooves in your shoulders. It causes neck pain. It can even lead to tension headaches. A properly fitted band should be parallel to the floor and tight enough that you can only fit two fingers underneath it. If it’s sliding up toward your shoulder blades, the math failed you.
British researcher Dr. Joanna Wakefield-Scurr at the University of Portsmouth has done extensive studies on breast displacement during exercise. Her work shows that most sports bras fail because the initial sizing is wrong. If the base measurement is off, the structural integrity of the garment can't hold up under movement.
How to Actually Measure Yourself
Grab a soft tape measure. Don't wear a padded bra while you do this; go braless or wear a very thin bralette.
First, measure your underbust. Pull it snug. Not "I can't breathe" tight, but firm. This is your base. If you get an odd number like 31, you'll likely need to try both 30 and 32 bands depending on the brand.
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Second, measure the fullest part of your bust. Don't squish yourself. Just let the tape rest.
Subtract the underbust from the bust. That's your cup size in inches.
If your underbust is 33 and your bust is 38, that’s a 5-inch difference. You’re looking for a 34DD or 34E. Ignore the "Plus Four" labels on the store walls. Trust the raw data of your own body.
Common Signs Your Math is Off
- The "Quadra-boob": If your breast tissue is spilling over the top of the cup, you need to go up at least one cup size.
- Floating Gore: The center piece of the bra (the gore) should sit flat against your sternum. If it's hovering, the cups are too small.
- The Band Slide: If you raise your arms and the bra moves up, the band is too big.
It’s also worth noting that your size will change. Monthly cycles, age, and activity levels all shift these numbers. A "size" is not a static identity; it's just a tool for a specific moment in time.
Taking Action on Your Measurements
Stop relying on the sizing charts provided by most major retailers. They are designed to fit you into their inventory, not to provide an accurate reflection of your anatomy. Instead, use your raw measurements to look for brands that offer a wider range of sizes, particularly those that use UK sizing standards, which tend to be more consistent for larger cup volumes.
Before you buy your next bra, do a "scoop and swoop." Lean forward, put the bra on, and manually move your breast tissue from the sides into the cups. You might find that the "perfect" cup size you just calculated suddenly feels a bit small. That’s because the tape measure can’t always account for tissue that has migrated toward your underarms due to years of wearing the wrong size.
Check your measurements every six months. If your weight fluctuates by even five or ten pounds, your cup size in inches is likely to shift. By staying on top of the actual math—not the industry's "Plus Four" version—you can finally find a fit that actually supports you.
Start by measuring your snug underbust today and use that as your firm band size. Subtract that directly from your standing bust measurement. Use that resulting number to shop at specialty boutiques or online retailers that acknowledge the 1-inch-per-letter rule. This simple shift in perspective is usually the difference between constant discomfort and a bra you actually forget you're wearing.