A B cup means your bust measures about 2 inches more than your band. On a 34 band that’s a bust of roughly 36 inches. Like every cup letter, B isn’t one fixed volume: a 38B holds about as much as a 34D, because cups grow with the band.
| Band 34 | Underbust over 32 and up to 34 inches (81–86 cm). We round your underbust up to the next even number. |
|---|---|
| Cup B | Bust about 2 inches bigger than the band: roughly 35½ to 36½ inches (90–93 cm) around the fullest part. |
| Sister sizes | 32C (tighter band) and 36A (looser band). Full diagonal: 28DD, 30D, 32C, 34B, 36A, 38AA. |
| Same size abroad | UK 34B · EU 75B · French 90B · Australia / NZ 12B |
Brands measure differently, and some US brands add inches to the band. Use these numbers as a starting point, then check the fit. Calculate your own size · See every sister size · Convert another size
B cup sizes across bands
| Size | Underbust (in) | Bust (approx. in) | Sister sizes |
|---|---|---|---|
| 30B | 28–30 | 32 | 28C, 32A |
| 32B | 30–32 | 34 | 30C, 34A |
| 34B | 32–34 | 36 | 32C, 36A |
| 36B | 34–36 | 38 | 34C, 38A |
| 38B | 36–38 | 40 | 36C, 40A |
| 40B | 38–40 | 42 | 38C, 42A |
| 42B | 40–42 | 44 | 40C, 44A |
Our guides to 32B, 34B and 36B go deeper on the most common B sizes.
Is a B cup small?
It’s often called small to medium, but the band changes everything. A 30B is small in volume; a 40B is closer to what many people picture as a full bust. Brastop’s sister size guide shows the pattern: going one band down and one cup up keeps the same cup volume.
The best bra styles for B cups
- Plunge: a low center adds shape and suits V necklines.
- Balconette: a lower top edge avoids gaping when you have less fullness on top.
- Lightly lined T-shirt bra: smooth under fitted tops.
- Bralette: comfortable for lounging; B cups often get enough support from a soft bralette.
- Push-up: if you want more cleavage. See our guide to push-up bras for small breasts.
Common B cup fit problems
| Problem | Likely fix |
|---|---|
| Gaps at the top of the cup | Balconette or plunge, or a smaller cup |
| Band rides up | One band down, one cup up (34B → 32C) |
| Center lifts off your chest | Plunge with a lower, narrower center |
| Straps slip | Racerback or straps set closer together |
The fit checker walks through each of these.
Shop B cup bras by size
Affiliate links-
32B bras
Look for plunge and T-shirt styles in a 32 band.
Search Amazon for 32b bras (opens in a new tab, affiliate link) -
34B bras
Very widely stocked in every style.
Search Amazon for 34b bras (opens in a new tab, affiliate link) -
B cup push-up bras
For extra lift and cleavage.
Search Amazon for b cup push-up bras (opens in a new tab, affiliate link) -
B cup bralettes
Soft, wireless support for smaller cups.
Search Amazon for b cup bralettes (opens in a new tab, affiliate link)
Results are Amazon’s, not our picks. Check the size range, the return policy and the features above before you buy.
How to confirm you’re a B cup
Measure snugly under your bust and round up to the next even number for the band, then measure loosely around the fullest part. A 2-inch difference is a B. The bra size calculator does it for you, and the C cup guide covers the next size up.
