B cup size: what it means, and why it changes by band
A B cup is two inches of difference between bust and band, on whatever band you wear. A 30B and a 40B share that two-inch gap, but the 40B holds more cup volume. The letter is the same in the UK, US, EU, France and Australia; our approximate Japan column prints C.
UK B: 2 in, 5.1 cm of difference
| 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 |
AU follows Berlei; Wacoal, Freya and Elomi keep UK FF and GG instead. JP is approximate because Japanese cup steps are 2.5 cm rather than one inch; our JP column stops at I. Wacoal's guide continues to JP K. "n/a" means outside this converter, not that the size does not exist. Cup-system definitions.
B cup size in inches and centimetres
A cup letter is a difference, not a volume. On a 34 band, a UK B is a 36 in (91.4 cm) bust. Every row is the band plus the cup's inches, the same arithmetic the calculator uses.
| Band | UK B bust |
|---|---|
| 30 in | 32 in (81.3 cm) |
| 32 in | 34 in (86.4 cm) |
| 34 in | 36 in (91.4 cm) |
| 36 in | 38 in (96.5 cm) |
| 38 in | 40 in (101.6 cm) |
| 40 in | 42 in (106.7 cm) |
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. Shown for UK B on a 34 band.
| 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 the +4 method does to this cup
Our +4 comparison rounds measurements to whole inches, adds 4 to an even underbust (5 to an odd one) and subtracts the padded band from the bust. Differences below 1 are labelled AA. For UK B (2 in) that changes the answer on every band.
| Underbust | Bust | Measured band (UK) | +4 method (US) |
|---|---|---|---|
| 30 in | 32 in | 30B | 34AA |
| 34 in | 36 in | 34B | 38AA |
| 38 in | 40 in | 38B | 42AA |
White and Scurr (2012) found the traditional method overestimated band size and underestimated cup size (Ergonomics 55(6)). A size from an older calculator may not be the size on this page.
B cup bras: which brand charts list one, by band
Some charts print individual sizes; others state band and cup ranges separately. Being inside both ranges does not confirm that the brand makes that combination. None of these checks confirms stock.
| Brand | UK B (2 in) |
|---|---|
| Elomi | Cup within guide range; bands 32 to 48 stated separately; combinations unconfirmed |
| Freya | Cup within guide range; bands 28 to 44 stated separately; combinations unconfirmed |
| Marks & Spencer | Chart entries: 28 to 44 |
| Victoria's Secret | International chart: 30 to 44. US guide: bands 30 to 46, cups A to O; combinations unconfirmed |
| The Little Bra Company | Cup within guide range; bands 28 to 38 stated separately; combinations unconfirmed |
| ThirdLove | Cup within guide range; bands 32 to 44 stated separately; combinations unconfirmed |
Elomi: US underband and cup sizing, 32 to 48 bands and B to O cups (UK B to K; its chart continues to UK KK). Freya: bands 28 to 44, UK cups B to K. Marks & Spencer: its UK row lists sizes from 28AA to 46K (no FF, HH or JJ); matched here by the label as printed. Victoria's Secret: US guide: bands 30 to 46, cups A to O, not every style in every size; international chart: bands 30 to 44, AA to DDD (UK E). The US ranges do not confirm every band/cup combination. The Little Bra Company: its size picker: bands 28 to 38, cups AA to D (a specialist for small sizes). ThirdLove: bands 32 to 44, AA to H in US labels, plus half cups; the ranges do not verify every combination or the UK pairing for G/H.
What is particular 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").
Is a B cup big?
The letter cannot say. A 34B holds the same cup volume as 32C and 36A, and the same as 30D two bands down: down a band, up a letter. So the volume of "a B cup" on a wide band can be worn as a D on a narrow one. We do not quote cup volumes or weights; no chart we cite gives them.
The +4 comparison changes a B to AA
A 34 in underbust and a 36 in bust give 34B on the measured-band method. Our +4 comparison pads the band to 38 in, leaving a difference of −2 in, and the below-one-inch AA category makes the answer 38AA. The same arithmetic gives 34AA at 30/32 and 42AA at 38/40. In our sister-size model each holds the same cup volume as the measured size, but on a band 4 in longer.
Marks & Spencer's US column calls it an A
On the M&S US size guide a UK 34B is a US 34A and a UK 36B is a US 36A, while a UK 32B keeps its letter (US 32B). Wacoal, Freya and Victoria's Secret all print UK B as US B, and so does this site. Buying M&S from the US, go by the UK label.
Where the charts start
Freya's and Elomi's printed cup ranges begin at B, so B is the smallest letter on either chart; Elomi's bands start at 32. The Little Bra Company, a specialist for small sizes, covers AA to D on bands 28 to 38. The coverage table above separates individual chart entries from broader guide ranges.
Quick answers
What does B cup mean?
Two inches of difference between your fullest bust and your snug underband, measured in the same units. The band number tells you the rest: a 30B and a 40B are different sizes.
What is a 34B in other countries?
UK and US 34B, EU 75B, France 90B and Australia 12B. Marks & Spencer's US column prints it as 34A.
What are the sister sizes of 34B?
32C on a band one size tighter and 36A on a band one size looser. All three hold the same cup volume.
Other sizes and tools
- Bra size calculatorMeasure your size with both methods shown
- UK to US converterAny size in any of six systems
- Sister size chartThe full diagonal chart and fit fixes
- 32BSister sizes, conversions and the measurements that land on 32B
- 34BSister sizes, conversions and the measurements that land on 34B
- A cupWhat the letter means, by band
- D cupWhat the letter means, by band
- DD cupWhat the letter means, by band
- E cupWhat the letter means, by band
- F cupWhat the letter means, by band
- G cupWhat the letter means, by band
- H cupWhat the letter means, by band
Sources
Each source was opened and checked on the date shown, or 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
- Marks & Spencer US, "Bras size guide": UK, AU/NZ, US/CA, EU and FR conversion table (opened 25 September 2026)
- Victoria's Secret Customer Care, "Size & Fit: International Size Chart (Outside the U.S.)": US sizes sold; bands 30 to 44, cups AA to DDD; US DDD = UK E (opened 25 September 2026)
- Freya, "International Bra Size Chart & Bra Sizing": band and cup tables in UK, EU, FR, AU/NZ, US and JP (opened 25 September 2026)
- Elomi, "International Size Guide": US underband and cup sizing, B to O cups and 32 to 48 bands; UK, EU, FR, AU/NZ, US and JP rows (UK cups to KK, US to P) (opened 29 September 2026)
- The Little Bra Company, "Fit Calculator": its size picker offers bands 28 to 38 and cups AA to D (opened 29 September 2026)
- Wacoal, "International Size Guide": UK AA to GG paired with US AA to J; AUS/NZ keeps FF and GG; band and cup rows are separate, not a stock list (opened 13 October 2026)
- Berlei Australia, size charts: AU band = underbust row, cups A to H with no FF or GG
- Victoria's Secret Customer Care, "Size & Fit: Bras & Bralettes": US guide states bands 30 to 46 and cups A to O, not all styles in all sizes; bralette rows print DD (E) and DDD (F) (opened 25 September 2026)