B cup size: what it means, and why it changes by band
A B cup is two inches of difference between bust and band, nothing more. A 28B and a 40B share a letter and nothing else: the actual amount of tissue they hold is not comparable.
B cup by band, in every system
| UK | US | EU | FR | AU | JP (approx.) |
|---|---|---|---|---|---|
| 30B | 30B | 65B | 80B | 8B | C65 |
| 32B | 32B | 70B | 85B | 10B | C70 |
| 34B | 34B | 75B | 90B | 12B | C75 |
| 36B | 36B | 80B | 95B | 14B | C80 |
| 38B | 38B | 85B | 100B | 16B | C85 |
| 40B | 40B | 90B | 105B | 18B | C90 |
"b cup size" gets about 18,100 Google searches a month in the US (Google Ads data, 26 September 2026).
Sister sizes keep the B cup's volume
Go down one band and up one letter, or up one band and down one letter, and the cup volume stays the same.
| UK | US | Compared with 34B |
|---|---|---|
| 32C | 32C | Tighter band, 1 letter up |
| 34B | 34B | This size |
| 36A | 36A | Looser band, 1 letter down |
The full diagonal chart, with fit fixes for a band that rides up or digs in, is on the sister sizes page.
What people get wrong about the B cup
"B cup" does not mean small
B is one inch more than an A and one inch less than a C, on whatever band you measure. A 40B holds far more volume than a 28B; the letter only describes the gap, never the size on its own (Wikipedia, "Bra size").
The traditional method can fail a B cup outright
A 34 in underbust and a 36 in bust give 34B on the measured-band method. The old +4 method pads the band to 38 in, two inches more than the bust itself, and returns no cup letter at all rather than a small one. See the worked 34B page.
Other sizes and tools
Sources
Each source was opened and checked on 25 September 2026. Where sources disagree, the page says so instead of picking one silently.
- Wikipedia, "Bra size": UK and US cup ladders, the +4 method, EN 13402 and JIS cup steps
- White J, Scurr J. Evaluation of professional bra fitting criteria for bra selection and fitting in the UK. Ergonomics 2012;55(6):704-711: 45 women; the traditional method overestimated band and underestimated cup