Bra sizing combines a number and a letter, and they are frequently read as size and volume. The relationship between them is the source of most sizing confusion.
The band carries the support
Most of the upward force supporting the bust comes from the band running around the torso, not from the straps over the shoulders.
That is why the band is fitted snugly and why a loose band rides up at the back, transferring load to the straps and producing shoulder discomfort.
The band number derives from a measurement taken around the ribcage beneath the bust, and it describes the circumference the elastic must grip.
The cup letter is a difference, not a volume
Cup size is calculated from the difference between the measurement around the fullest part of the bust and the band measurement.
Because it is a difference, the same letter represents a larger absolute volume on a larger band and a smaller one on a smaller band.
This is the single most misunderstood feature of the system, and it is why a cup letter cannot be interpreted without knowing the band it accompanies.
Sister sizes follow from the arithmetic
Moving down one band size while moving up one cup letter preserves approximately the same cup volume with a tighter band.
Fitters use this relationship when a cup fits correctly but the band does not, adjusting both together rather than changing one alone.
It also explains why someone convinced they wear an unusual cup letter frequently turns out to be wearing a band several sizes too large.
Shape varies independently of size
Two people with identical measurements can require different styles, because tissue distribution, root width and projection differ.
Cut-and-sewn cups, molded cups and different wire shapes each accommodate different distributions, which is why fit varies across styles from the same maker.
No measurement captures this, which is the reason fitting remains a hands-on process rather than something a formula resolves completely.
Sizing is not standardized between brands
There is no enforced standard defining what a given band and cup must measure, so each manufacturer works to its own block.
Wire width, band elasticity and cup depth all vary, and the differences are large enough to shift a wearer a full size between brands.
Anyone experiencing pain, marking or persistent discomfort should be fitted in person, since those symptoms usually indicate a fit problem rather than a size that needs enduring.