Bra Sister Size Calculator
Pick your current UK size from the dropdown and you’ll see the sizes that share the same cup volume on a wider or narrower band — handy when your go-to is out of stock.
What are sister sizes?
Sister sizes share the same cup volume but have different band lengths. Going up a band size means going down a cup letter — and vice versa. This is useful when your preferred size is out of stock, or you need a different fit around the ribcage.
When Should You Use a Sister Size?
Sister sizing is most useful in two situations. The first: your measured size is unavailable — a particular style doesn't come in your exact combination, or it's out of stock in your size. The second: the band on a specific bra style runs unusually tight or loose relative to the cup. Going up one band size (and dropping one cup letter) or down one band size (and going up one cup letter) can give you a perfect fit when the original label size doesn't.
One important caveat: sister sizes are not perfect substitutes. The band width changes, which affects how the straps sit and how much support the band provides. Ideally, wear your true size and only use sister sizing as a temporary fix or to handle unusual brand sizing.
Are Sister Sizes the Same Cup Volume?
Yes — by definition, sister sizes contain the same cup volume. A 32DD, a 34D, and a 36C all hold the same amount of breast tissue. This is because UK cup sizes are not absolute measurements; they are the difference between your bust and underbust.
Since the cup letter changes by one step for every band size you move, the actual volume stays constant. This is also why a woman measured as a 34D might be surprised to discover she fits a 32DD just as well — the cup holds the same volume, just anchored to a narrower band.
UK Sister Size Chart (by Band)
Each row shows sizes with the same cup volume. Move left for a tighter band + bigger cup; move right for a looser band + smaller cup.
| ← Smaller Band | Your Size | Larger Band → |
|---|---|---|
| 32B | 34A | 36AA |
| 32C | 34B | 36A |
| 32D | 34C | 36B |
| 32DD | 34D | 36C |
| 32E | 34DD | 36D |
| 32F | 34E | 36DD |
| 32FF | 34F | 36E |
| 32G | 34FF | 36F |
| 34G | 36FF | 38F |
| 34GG | 36G | 38FF |
| 34H | 36GG | 38G |
Frequently Asked Questions About Sister Sizes
What are bra sister sizes?+
They're sizes that hold the same volume of breast tissue but on a different band. A 34C, a 32D and a 36B are all sister sizes — the cups contain the same amount, only the band width changes. It's the quickest trick for getting a comfortable fit when your normal size isn’t available.
How does sister sizing work in the UK?+
Straightforward maths: drop one band number (say 34 to 32) and go up one cup letter (C to D). Or jump up a band (34 to 36) and drop a letter (C to B). The volume stays the same — you're just redistributing it between band and cup.
When should I use a sister size?+
Two situations come up all the time. First, your calculated size is sold out and you need an alternative right now. Second, the band on a particular bra style runs tight or loose, but the cup itself fits. Going up or down one band (and adjusting the cup letter) can fix that.
Is a 34D cup the same size as a 36D?+
No. A 36D has more cup volume than a 34D. Cup letters aren’t fixed amounts — they scale with the band. That’s exactly why sister sizing exists: to let you swap band width without accidentally changing how much the cup holds.
How do I find my sister sizes?+
Pick your UK size in the calculator above. It’ll list the next size up (wider band, one cup letter smaller) and the next size down (tighter band, one cup letter larger). Those are your closest matches.