Sister Size Bra Calculator
Find every bra size that holds the same cup volume as yours. Perfect when your size is out of stock.
Select Your Current Size
Band Size
Cup Size
📉 Smaller Band (Tighter)
📈 Larger Band (Looser)
ℹ️ When to Use Each Option
The cup volume stays the same but the band sits firmer. Try this if your exact size is sold out but you need firm support.
Looser around the ribcage. Useful if your exact size is sold out and the band of your current bra feels too snug.
Sister sizes are NOT perfect substitutes. A different band changes how straps sit and how much the band supports. Always try to wear your true measured size.
📐All Sizes with Equal Cup Volume to 34C
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.
Sister Sizing FAQs
What is a bra sister size?+
Sister sizes are groups of bra sizes that share the exact same cup volume, even though their band numbers and cup letters are different. For example, a 34C and a 36B have the same cup volume, but the 36B has a wider band.
How does sister sizing work?+
The rule is: if you go down one band size (e.g. from 34 to 32), you must go up one cup letter (e.g. from C to D) to maintain the same cup volume. Conversely, if you go up one band size (from 34 to 36), you must go down one cup letter (from C to B).
When should I wear a sister size?+
You should use a sister size if your calculated size is out of stock, or if your bra band feels slightly too tight or too loose but the cup volume fits you perfectly. For example, if your 34C cup fits but the band is too snug, try a 36B.
Is a 34D cup the same size as a 36D?+
No. A 36D is one cup volume larger than a 34D. Cup sizes are not absolute; they are relative to the band size. A D cup on a 36 band holds more volume than a D cup on a 34 band.