B Cup Size Chart: 30B to 40B and 70B to 85B Explained
A B cup size chart shows how B cup changes across different band sizes, such as 30B to 40B or 70B to 85B. B cup is not one fixed breast size because cup volume changes with the band, even when the cup letter stays the same.
What does a B cup size chart show?
A B cup size chart shows how the band size, cup letter, cm equivalent, sister sizes and fit notes work together.
It helps you understand why 30B, 32B, 34B, 36B, 38B and 40B all share the letter B, but do not have the exact same cup volume. It also explains common cm-based labels such as 70B, 75B, 80B and 85B, which are often used in SG/MY and Asian or EU-style size charts.
B Cup Size Chart: 30B to 40B
A B cup usually means the bust is about 5 cm or 2 inches larger than the underbust, but the cup volume increases as the band size increases.
| Bra size | Common cm equivalent | Band meaning | Cup meaning | Sister sizes | Fit notes |
|---|---|---|---|---|---|
| 30B | 65B / sometimes close to 70A | Smaller band size | About 5 cm / 2 in bust-band difference | 28C / 32A | Less common in stores; may need sister sizing |
| 32B | 70B | Small-to-medium band size | About 5 cm / 2 in bust-band difference | 30C / 34A | Common smaller B cup size |
| 34B | 75B | Medium band size | About 5 cm / 2 in bust-band difference | 32C / 36A | Common everyday B cup size |
| 36B | 80B | Larger band size | About 5 cm / 2 in bust-band difference | 34C / 38A | More cup volume than 34B |
| 38B | 85B | Larger band size | About 5 cm / 2 in bust-band difference | 36C / 40A | More cup volume than 36B |
| 40B | 90B | Largest band in this chart | About 5 cm / 2 in bust-band difference | 38C / 42A | Largest B cup volume in this chart range |
Size conversion can vary by brand, so this chart is a guide rather than a fixed rule. Always compare your measurements with the brand’s own size chart before buying.
70B, 75B, 80B and 85B: What do they mean?
70B, 75B, 80B and 85B are common cm-based bra sizes used in many Asian and EU-style size charts.
The number usually refers to the band size range, while B refers to the cup letter. In many SG/MY shopping contexts, 32B is often close to 70B, 34B to 75B, 36B to 80B and 38B to 85B, but each brand may convert sizes slightly differently.
| Inch-style size | Common cm equivalent | Simple meaning |
|---|---|---|
| 32B | 70B | Smaller band with B cup |
| 34B | 75B | Medium band with B cup |
| 36B | 80B | Larger band with B cup |
| 38B | 85B | Larger band with more B cup volume |
Is 30B smaller than 34B?
Yes, 30B is smaller than 34B because it has a smaller band and less cup volume.
The letter B does not mean the same cup volume on every band size. A 30B, 32B and 34B all have a B cup difference, but 34B has more cup volume than 32B, and 32B has more cup volume than 30B.
Is 34B the same as 75B?
34B is commonly close to 75B in many Asian and EU-style size systems, but it is not guaranteed to fit exactly the same across every brand.
If you usually wear 34B, you may often see 75B as the nearby cm-based option. Still, check the product’s own size chart because band stretch, cup shape and conversion rules can differ.
Why B cup is different on every band size
B cup is different on every band size because cup volume scales with the band.
Sister sizing explains this clearly: when you go down one band size, you usually go up one cup letter to keep a similar cup volume. When you go up one band size, you usually go down one cup letter.
| Original size | Sister size down | Sister size up | What it means |
|---|---|---|---|
| 32B / 70B | 30C / 65C | 34A / 75A | Similar cup volume with different band fit |
| 34B / 75B | 32C / 70C | 36A / 80A | Useful if 34B cup feels right but band does not |
| 36B / 80B | 34C / 75C | 38A / 85A | Helps adjust band comfort while keeping similar volume |
| 38B / 85B | 36C / 80C | 40A / 90A | Helpful when the cup is close but the band needs changing |
How to use this chart before buying a bra
Use this B cup size chart as a starting point, then compare your own measurements with the brand’s chart before choosing.
- Measure your underbust snugly around the ribcage.
- Measure the fullest part of your bust while keeping the tape level.
- Check whether your bust-band difference is close to a B cup.
- Compare your band range with the brand’s size chart.
- Check whether the band feels secure without digging or riding up.
- Check whether the cups have gaping, overflow or pressing.
- Try a sister size if the cup feels close but the band feels too tight or loose.
For everyday comfort after checking your B cup size
For everyday comfort, a seamless or wireless bra can help the band and cups sit smoothly under daily outfits without feeling overly structured.
Seamless Bra Collection
Good for a smooth invisible fit, breathable comfort and soft daily support.
- Smooth under T-shirts and fitted tops
- Soft feel for daily wear
- Best for a clean everyday outline
Wireless & Everyday Bra Collection
Good for gentle daily support when you prefer a lighter, easy-wear feel.
- Wireless and comfort-focused options
- Breathable styles for daily support
- Best for long days, work and casual outfits
FAQ
Q1: What is B cup size in cm?
A: B cup size usually means the bust is about 5 cm larger than the underbust measurement.
Q2: Is 34B the same as 75B?
A: 34B is commonly close to 75B in many Asian and EU-style size charts, but sizing can vary by brand.
Q3: Is 36B the same as 80B?
A: 36B is commonly close to 80B in many cm-based size charts, but the exact fit depends on the brand’s own conversion.
Q4: Is 38B the same as 85B?
A: 38B is commonly close to 85B in many cm-based size charts, but brand sizing, fabric stretch and cup shape can affect fit.
Q5: Is 30B smaller than 32B?
A: Yes, 30B is smaller than 32B because it has a smaller band and less cup volume.
Q6: Does B cup size look the same on every band?
A: No, B cup size does not look the same on every band. A 30B, 34B and 40B all use the B letter, but their cup volumes are different.
