B Cup Size
A B cup is a bust measurement 2 inches larger than the band. That is all a cup letter says: which is why a B cup on a 30 band and a B cup on a 40 band are very different bras.
What is a B cup?
A B cup means the fullest bust measurement is 2 inches (≈ 5 cm) larger than the underbust band. In UK sizing the same 2-inch difference is written B; in EU sizing B. It is not a fixed volume: the volume a B cup holds grows with the band it sits on.
Same letter, nine volumes. A 28B and a 44B are both “B cups”: the second holds several times the volume of the first.
Every B-cup size
The same 2-inch difference on each band. Read across for measurements, sister sizes and the EU / FR / AU labels; tap a size for its full guide.
| Size | Underbust | Bust | Sister sizes | EU · FR · AU |
|---|---|---|---|---|
| 28B | 27–28″ · 69–71 cm | 30″ · 76 cm | — · 30A | 60B · 75B · 6B |
| 30B | 29–30″ · 74–76 cm | 32″ · 81 cm | 28C · 32A | 65B · 80B · 8B |
| 32B | 31–32″ · 79–81 cm | 34″ · 86 cm | 30C · 34A | 70B · 85B · 10B |
| 34B | 33–34″ · 84–86 cm | 36″ · 91 cm | 32C · 36A | 75B · 90B · 12B |
| 36B | 35–36″ · 89–91 cm | 38″ · 97 cm | 34C · 38A | 80B · 95B · 14B |
| 38B | 37–38″ · 94–97 cm | 40″ · 102 cm | 36C · 40A | 85B · 100B · 16B |
| 40B | 39–40″ · 99–102 cm | 42″ · 107 cm | 38C · 42A | 90B · 105B · 18B |
| 42B | 41–42″ · 104–107 cm | 44″ · 112 cm | 40C · 44A | 95B · 110B · 20B |
| 44B | 43–44″ · 109–112 cm | 46″ · 117 cm | 42C · — | 100B · 115B · 22B |
The B cup, in numbers
Volume is the pairing, not the letter. On a 34 band a B cup holds the same as a 32C and a 36A: one letter away on the neighbouring bands. Read across any sister row and the letters change while the volume doesn't.
Is a B cup big?
Only in relation to a band: a B cup is a modest cup on any band, and on larger bands it is a fuller bust than the letter suggests.
The honest way to talk about size is the pair (32B, 36B, 40B) because the letter measures a difference, not an amount. This is also why sister sizes exist: move down a band and up a letter and the volume stays the same.
Key takeaway: “B cup” describes a 2-inch difference. Volume depends on the band; the letter alone is not big or small.
What people get wrong about B cups
Every symptom and its fix: bra fit guide.
Wearing a B cup on a band that rides up?
The old +4 rule inflates the band by four inches and understates the cup by two letters. If you were sized that way, your B cup is probably really a D cup on a smaller band.
| If a +4 chart said | Snug-measure size is likely | Same volume? |
|---|---|---|
| 32B | 28D | Yes: the band supports instead of the straps |
| 34B | 30D | Yes: the band supports instead of the straps |
| 36B | 32D | Yes: the band supports instead of the straps |
| 38B | 34D | Yes: the band supports instead of the straps |
| 40B | 36D | Yes: the band supports instead of the straps |
B cup in UK, EU and other systems
2″ difference. Matches UK and EU letters.
Identical to US. Full-bust specialists (Panache, Freya, Curvy Kate) label in UK letters.
Identical to US. FR, IT, DE and JP use EU cup letters.
Convert any exact size with the size converter.
Bra styles that fit a B cup
Shallow-to-modest cups suit plunge, demi and balconette shapes that create lift and shape without excess fabric; wireless and bralette styles also work because there's little weight to support. Avoid full-coverage cups cut for projection: they tend to gape at the top on a shallow profile.
Frame like a 28 band, also look for narrower wires and shorter straps The same B cup is deeper and narrower here.
The widest choice of styles at this cup. Molded for a smooth line, seamed for shape and lift.
The B cup is wider and shallower; prioritise a firm, wide band and a wire that fully encloses the root. Longline and three-hook styles help.
Shape matters as much as size: see the fit guide for the five breast shapes and the constructions that suit each.
B cup: answered
How big is a B cup?
A B cup is a bust 2 inches (5 cm) larger than the band. Its volume depends on the band: a 30B is far smaller than a 40B.
What is a B cup in UK sizing?
A US B cup is a UK B cup: the same letter.
What is a B cup in European sizing?
A US B cup is an EU B cup. EU letters continue the alphabet one letter per inch above D.
What is the difference between a B cup and a C cup?
One inch of bust-band difference: B is 2″, C is 3″. On the same band, C holds one cup step more volume.
Is a 34B the same as a 36A?
They are sister sizes: the same cup volume on a band two inches longer. The 36A will feel looser in the band and offer less support.
What measurements make a B cup?
Any bust measurement 2 inches above the snug underbust. Examples: underbust 31″ / bust 34″ → 32B; underbust 35″ / bust 38″ → 36B.
Opens the matching HauteFlair collection; use your measured size there. HauteFlair is owned by the same company as Brasize.ai; that relationship never changes the sizing math, charts, or fit advice on this page. How this site makes money.
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A cup letter is only half the answer. Two measurements give you both halves.
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