Why does my “usual” size feel different in different bra styles or brands?

The bra sizing system is unintuitive and quite complex. A bra size consists of a band size (number) and a cup size (letter). The cup letter isn’t a fixed volume, it’s a ratio, the difference in centimetres between the measurement under the bust and the measurement over the bust. When the band size changes, the cup letter has to compensate to keep the bust volume the same (so a D cup on a 75 band is smaller in actual volume than a D cup on an 85 band). On top of that, different brands cut their patterns differently, which means the same actual size can correspond to different labelled sizes between brands.

How the difference in circumference becomes a letter:
If a customer has an underbust of 78 cm (rounded to band 80) and an overbust of 96 cm, the difference is 18 cm → C cup → size 80C.

Same cup letter, different actual volume
This is the part that makes the system especially counterintuitive: since the cup only describes the difference in cm, not an absolute volume, the same letter means different things depending on the band.
70D: underbust ~70 cm, overbust ~88 cm (difference 18 cm) 85D: underbust ~85 cm, overbust ~103 cm (difference 18 cm)
Both are a “D”, but the actual bust volume (and the cup’s physical size/fabric area) is considerably larger on the 85 band, the cup simply has to cover a much larger circumference at the same “excess”.

Sister sizing
Since cup volume scales with the band, manufacturers usually construct their patterns so you can go up one band and down one letter (or the other way round) and land on a cup with roughly the same actual volume, useful to know if you’re stuck between sizes:

Updated on: 09/09/2026