Bra cup sizes explained
By BraSizeMatch · Practical fit guidance
A cup letter only means something next to its band. Compare cup sequences and learn how relative volume works.
A cup letter is a relationship, not a volume
A cup compares bust to band. Each step up the alphabet within one band adds a bit of cup volume, but the same letter on a larger band holds more than on a smaller band. A 32D and a 38D are not the same cup.
Cup sequences by system, band 34 example
Read each row as the same relative step: the number counts cups above AA. US, UK and EU agree early and then diverge.
| Step above AA | US | UK | EU |
|---|---|---|---|
| 1 | A | A | A |
| 2 | B | B | B |
| 3 | C | C | C |
| 4 | D | D | D |
| 5 | DD | DD | E |
| 6 | DDD | E | F |
| 7 | G | F | G |
| 8 | H | FF | H |
| 9 | I | G | I |
| 10 | J | GG | J |
Relative volume and sister sizes
Going down one band and up one cup keeps approximate volume, which is the idea behind sister sizes. It is approximate: wire width, strap placement and tension change too.
Larger cups are therefore not just bigger, they interact with the band. If a cup feels wrong, ask whether the band is also doing its job before changing the letter.
Common label traps
Some brands call a step DDD, others F. UK adds double letters such as FF and GG. Check the brand’s chart before assuming a letter matches another system, and see the fit check to judge cups in the mirror.
Find a starting size
If your cups gape or overflow, size and cup shape may both matter. Check your current measurements before changing styles, then compare the estimate with how your bra actually fits.