How bra cup sizes actually work

The most common misunderstanding about bra sizing is treating the cup letter as if it described an absolute size — “a C cup” as a fixed thing, the same on everyone. It isn’t. A cup letter only means something relative to the band size next to it.

The method: two measurements, one subtraction

1. Underbust (band): measured snugly around the ribcage, directly under the bust.
2. Bust: measured around the fullest point, tape level, not pulled tight.
3. The difference between the two, in inches, is the cup step. One inch of difference is roughly one cup letter: 1”=A, 2”=B, 3”=C, 4”=D, and so on.

That’s the whole mechanism. A 32-inch band with a 35-inch bust (3” difference) is a C cup. A 38-inch band with a 41-inch bust — also a 3” difference — is also a C cup, on a much larger frame. The letter never describes a fixed cup volume; it describes how the bust compares to the band underneath it.

Band size: the modern method vs. the old one

Band size itself is close to a direct reading of the underbust measurement, rounded to the nearest even number (32, 34, 36…). This is the method used throughout our bra size converter.

An older convention still printed by some brands adds 4–5 inches to the underbust measurement before rounding. The two methods can land on different band sizes from the same underbust measurement — which is the single most common reason someone’s self-measured size doesn’t match what a specific brand’s own chart recommends. If a brand’s fitting guide gives you a noticeably different band than a converter does, it’s very likely still using the older method rather than making an error.

Why the EU number looks so different

EU/France/Italy/Spain band sizes use a completely different number for the same measurement — roughly the underbust in centimetres, rounded to a multiple of 5. A US/UK 34 is a EU 75; a US/UK 38 is a EU 85. The cup letters mostly line up between the two systems for the smaller sizes, and diverge more at the largest cup sizes, where US brands sometimes use DDD/F/G where a UK or EU brand would use a single-letter step.

Why brands still disagree with the calculation

Two people with identical underbust and bust measurements can genuinely need different sizes in different brands, because “cup shape” — how that volume is distributed front-to-back and side-to-side — isn’t captured by a band-and-cup number at all. The method above gives a reliable starting point for narrowing down a size to try, not a guarantee, which is exactly why band and cup are the beginning of a fitting, not the end of one.

Enter your own underbust and bust measurements into the bra size converter to get the matching US/UK and EU size.