Last updated: August 2026
Comparing 30A and 30B: they differ in band width, cup volume, or both. Below is exactly how they compare and how to work out which one is right for your body.
30A vs 30B at a glance
| 30A | 30B | |
|---|---|---|
| Band (underbust) | 29–30 in (74–76 cm) | 29–30 in (74–76 cm) |
| Bust | 31–32 in (79–81 cm) | 32–33 in (81–84 cm) |
| Cup (US) | A | B |
| UK | 30A | 30B |
| EU | 65A | 65B |
The band difference
Both sit on a 30 band, so they fit the same ribcage — an underbust of about 29–30 inches. Everything that separates them is in the cup.
The cup difference
On its own band, 30B takes the larger cup letter. Remember the letter is a bust-minus-band difference, though, so cup volume also depends on the band each letter sits on.
So which one is actually bigger?
30A and 30B are genuinely different fits — the cup volumes are not the same, so this is not a simple sister swap. The only reliable way to choose is from your own measurements; the bra size calculator will point you to the right one.
When to choose 30A over 30B
- Choose 30A if your measurements match its underbust of 29–30 in and bust of 31–32 in.
- Choose 30B if you are nearer its underbust of 29–30 in and bust of 32–33 in.
How to check the fit is actually right
Numbers get you to the right size on paper; these hands-on checks tell you whether it fits in real life. Do them in front of a mirror, wearing the bra on the loosest hook:
- Band: it should sit level all the way around, in line with the front. Slide two fingers under it at the back — snug, not loose, no riding up when you raise your arms. The band does roughly 80% of the support work, so this matters most.
- Cups: do a quick “scoop and swoop” — lean forward, settle everything into the cup, then stand up. There should be no spillage over the top or sides, and no wrinkling or gaping at the edge. Spillage means size up a cup; gaping means size down a cup.
- Centre gore: the little bridge between the cups should lie flat against your breastbone. If it floats off the body, the cup is usually too small.
- Straps: you should fit a finger under them comfortably. If they dig in or carry most of the weight, the band is too loose — tighten it or size the band down.
30A sister sizes
| Size | Underbust | Bust |
|---|---|---|
| 28B | 27–28 in | 30–31 in |
| 30A | 29–30 in | 31–32 in |
| 32AA | 31–32 in | 32–33 in |
30B sister sizes
| Size | Underbust | Bust |
|---|---|---|
| 28C | 27–28 in | 31–32 in |
| 30B | 29–30 in | 32–33 in |
| 32A | 31–32 in | 33–34 in |
International conversions
30A converts to 30A (UK) and 65A (EU); 30B converts to 30B (UK) and 65B (EU). Cup letters above D differ between US, UK and EU systems, so always check a brand’s own country of sizing.