You finally figure out your bra size. You go to buy it. It's sold out everywhere, doesn't exist in stores near you, or costs more than your electric bill. Sound familiar?
This is where sister sizing saves you. It's not a workaround or a compromise — it's actually how bra sizing works, and once you understand it, you'll never be stuck again.
Sister sizes are bra sizes that contain the same cup volume but with a different band size. Go up a band size and you go down a cup size. Go down a band size and you go up a cup size. The breast tissue being held is the same — only the band changes.
Cup letters don't mean a fixed size — they represent a ratio between your band and your bust. A D cup on a 32 band is very different from a D cup on a 42 band. The letter only makes sense in context of the number next to it.
This is why women who lose or gain weight often feel like their cup size changes dramatically — the ratio shifts even if actual breast volume doesn't change that much.
Each row contains sister sizes — same cup volume, different band. Your size is interchangeable with the sizes on either side in the same row.
| Smaller Band | Your Size | Larger Band |
|---|---|---|
| 32D | 34C | 36B |
| 32DD | 34D | 36C |
| 32E | 34DD | 36D |
| 34E | 36DD | 38D |
| 34F | 36E | 38DD |
| 36F | 38E | 40DD |
| 36G | 38F | 40E |
| 38G | 40F | 42E |
| 38H | 40G | 42F |
| 40H | 42G | 44F |
Use it when your exact size is unavailable, when you're between sizes, or when a specific style only comes in limited sizing. A smaller band feels tighter and provides more support. A larger band feels looser and may ride up if too relaxed for your body.
For women in extended sizes — F cup and above — sister sizing expands your options significantly since availability drops fast in physical stores. If you're a 38H and it's unavailable, a 40G or 36I are your sisters. Same cup volume, three times the search options.
Sister sizing works for volume only. It doesn't fix issues with cup shape, wire width, or strap placement. If a bra's underwire is too narrow for your breast root, a sister size won't solve that — you need a different brand or style.
Don't know your starting size yet?