Bra Cup Sizes Explained: What Do AA, A, B, C and D Really Mean?

Quick Answer

Cup letters describe the difference between your band measurement and your full bust measurement, not a fixed breast size. AA is roughly a half-inch difference, A about one inch, B about two, C about three and D about four. Because that difference grows as band size grows, a 34B and a 38B hold different volumes even though the letter matches.

Cup letters confuse people because they look like clothing sizes, where a medium is a medium. They aren't. A cup letter is shorthand for a measurement gap — the distance between your ribcage and the fullest part of your bust — so the same letter means something different on a 32 band than on a 40.

That's why two women can both wear a B cup and look nothing alike, and why a bra that fit last year can stop fitting after a five-kilo change. Here's what each letter actually represents, and how to use that when you're choosing a bra.

How Cup Letters Are Calculated

Measure your underbust snugly around your ribcage. That number, rounded to the nearest even number, is your band size. Then measure around the fullest part of your bust, keeping the tape level. Subtract the band from the bust. Each inch of difference equals one cup letter, starting at A for roughly one inch.

This is why the band comes first. If your underbust is 34 inches and your bust is 37, you're looking at a 34C. The letter alone tells you nothing useful — it only becomes a size when paired with a band number.

• Measure without a padded bra; a thin unlined bra or no bra gives the most accurate reading.
• Keep the tape parallel to the floor at the back — a tape that rides up inflates the bust number.
• If you land between sizes, try the smaller band with the larger cup first.

What AA, A, B, C and D Mean in Practice

AA is around a half-inch difference and is the smallest cup size stocked by most brands. A is roughly one inch, B around two, C around three and D around four. Beyond D, sizing continues with DD, E, F and so on, but the same principle applies: each step adds about an inch of difference.

The critical point is that a 32D and a 38D are not the same size. The 38D has a wider band and a larger cup overall, because the four-inch difference is measured against a bigger frame. This is why 'sister sizing' works — going down a band and up a cup (or the reverse) keeps a similar cup volume.

• Sister size example: 34C is close in volume to 32D and 36B.
• If a band feels tight but the cup fits, try one band up and one cup down before assuming the bra is wrong.

Why the Same Letter Fits Differently Across Brands

There's no single legal standard for cup grading, so a B in one brand can feel like an A or a C in another. Some brands grade cups more generously, some run shallow, and some change their grading when they move production. This is normal, not a sign that your body changed.

The practical fix is to treat your size as a starting point, not a rule. When you try a new brand, order your usual size plus one sister size, and judge by how the bra behaves rather than the label.

• Check the brand's own size chart rather than assuming your usual letter carries over.
• Read reviews that mention cup depth — they're often more useful than the size chart.

Signs Your Cup Size Is Wrong

A cup that's too small shows up as spillover at the top or sides, wires that sit on breast tissue instead of flat against the ribcage, and a centre gore that won't lie flat. A cup that's too large usually gapes at the top, wrinkles in the fabric, and lets the band ride up because there's no bust weight holding it down.

Both problems can look like a band problem. Before you change band size, check the cup: if the gore floats or the wires dig into tissue, the cup is the likely culprit.

• Scoop your breast tissue into the cup after fastening — it changes the fit more than most people expect.
• If one side is larger, fit to the larger side and adjust the strap on the smaller side.

How Cup Size Changes Over Time

Cup size isn't permanent. Weight changes, pregnancy, breastfeeding, hormonal shifts and age all affect breast tissue and ribcage measurement. A bra that fit six months ago may now need a different cup even if the band still feels fine.

Rather than replacing your whole drawer at once, refit every six to twelve months and replace the bras that no longer pass the gore-and-wire check. It's cheaper than buying a full set every time something shifts.

• Refit after any change of more than about three kilos.
• During pregnancy and nursing, expect cup size to change more than once — choose styles that accommodate that.

Quick Cup Size Reference

AA cup

About a half-inch difference between band and bust. The smallest cup most brands stock. If an A cup gapes badly, try AA before dropping a band size.

A cup

Roughly one inch of difference. Often the starting point for smaller busts. A shallow A can look like a B in brands that grade generously.

B cup

About two inches of difference — the most commonly worn cup size, and the one most affected by brand grading. Try your usual B plus one sister size when switching brands.

C cup

Around three inches of difference. A 34C holds a different volume than a 38C. Judge fit by the gore and wires, not the letter.

D cup

About four inches of difference, and the point where many brands extend into DD and beyond. If the gore floats or wires sit on tissue, go up a cup before up a band.

Cup Sizes in Real Bras

• Dahlia Lace Full Cup Bra - Black: Full cup coverage with no padding, wired, wider shoulder straps and a side sling, in soft floral lace. A full cup style holds more of the bust inside the cup, which suits fuller cups where a lower-cut bra tends to spill. The wider straps and side sling add support without padding, so the cup shape stays natural.
• Radiate Lace Wirefree Bra - Lavender Spritz: Wirefree construction in a purple and yellow lace design, available in back and cup sizes. If wires dig into your breast tissue because the cup is slightly off, a wirefree style removes that pressure point entirely — useful when you're between cup sizes or want a softer feel.
• Maternity Bra (Leakproof) - Crystal: Firm fit for extra support and shape, particularly for a fuller cup, with a Dry Layer that absorbs breast milk and a waterproof outer layer. Cup size often changes during nursing, so a firmer fit with a fuller cup gives more room to adjust without losing support — and the leakproof layers mean you're not layering pads on top.
• Lace Midi Brief - White: Low rise midi brief with lace panels at the front, full bottom coverage and a cotton gusset, sold by size. Knicker sizing runs independently of cup size, so match your brief to your hip measurement rather than your bra letter — the two don't move together.

Cup letters are a measurement shorthand, not a verdict on your body. Measure the band-to-bust difference, treat the letter as a starting point, and let the gore, wires and coverage tell you whether the cup is actually right.