The cup letters count inches: A is 1, B is 2, C is 3. This page is about B only: what it measures, how it changes with the band, and how to tell whether B is really your letter.
What a B cup measures
Measure snugly around the ribcage just under the bust. That sets the band. Then measure loosely around the fullest part of the bust while standing. If the second number is about 2 inches larger, the cup is B.
A 2-inch difference is the same in every country’s system, so a UK 34B, a US 34B and an Australian 12B are one size. In European sizing the band changes to centimeters (34 becomes 75) but the letter stays B, so the same size is a 75B.
B cup measurements for every band
| Band | US size | Bust about | UK | EU | AU | Sister sizes |
|---|---|---|---|---|---|---|
| 28 | 28B | 30 in (76 cm) | 28B | 60B | 6B | 30A |
| 30 | 30B | 32 in (81 cm) | 30B | 65B | 8B | 28C, 32A |
| 32 | 32B | 34 in (86 cm) | 32B | 70B | 10B | 30C, 34A |
| 34 | 34B | 36 in (91 cm) | 34B | 75B | 12B | 32C, 36A |
| 36 | 36B | 38 in (97 cm) | 36B | 80B | 14B | 34C, 38A |
| 38 | 38B | 40 in (102 cm) | 38B | 85B | 16B | 36C, 40A |
| 40 | 40B | 42 in (107 cm) | 40B | 90B | 18B | 38C, 42A |
| 42 | 42B | 44 in (112 cm) | 42B | 95B | 20B | 40C, 44A |
| 44 | 44B | 46 in (117 cm) | 44B | 100B | 22B | 42C |
The bust column is a guide. Bands round to an even number, so the real gap can be a little over or under 2 inches. If you are between two letters, the bra size calculator can use a leaning measurement to decide.
Same B, different volume
A B on a 30 band and a B on a 40 band are not the same amount of cup. Each band size up adds roughly one cup’s worth of volume, so a 40B holds about what a 34DD holds on a smaller frame. That is why two people who both “wear a B” can look completely different.
B is also a letter people are often put in by mistake. The old add-4-inches method turns a 30-inch underbust and a 36-inch bust into a 34B, when the measurements say 30DDD (UK 30E): a much firmer band and a much bigger letter. If the band of a B bra rides up at the back, try the sister size one band down and one cup up, and measure again without the extra inches.
Styles that suit a B cup
Most bra styles are made in B, so the choice is about shape, not availability.
- T-shirt bras with molded cups give a smooth line under fitted tops. They suit B cups that are evenly full.
- Plunge bras suit B cups that are wider set or fuller at the bottom, because the low center lets the cups sit closer to the body.
- Bralettes and soft cups work well in B, since there is less weight to lift and sizing is often S, M, L.
- Balconettes give a rounder shape at the top if the cup otherwise gapes there.
If the top of the cup stands away from the breast, the cup shape is wrong rather than too big. See the fit check before going down to an A.
Signs B is not your letter
- Spilling over the top or sides: try C on the same band.
- Wrinkles across the whole cup, not just the top: try A on the same band, or the sister size up a band and down a letter.
- The center panel lifts off the chest: the cups are too small, even if they look fine from the front.
- The band rides up but the cups fit: go down a band and up a cup, for example 34B to 32C.
B cup in other countries
B is one of the few letters that never changes name. US, UK, Australian, European and French sizes all call it B. Only the band number changes: a 34B is a UK 34B, an AU 12B, an EU 75B and a French 90B. For the full conversion, use the bra size conversion chart.
Next letter up: what is a C cup.
Sources & method: sizing rules follow the standard band-and-cup system used by UK, US, AU and EU brands, checked against fitting guides from specialist retailers and fitting communities. How we work.