A sister size holds the same cup volume as your true size but with a different band number. Going up a band size while going down a cup letter (or the reverse) keeps the volume the same.
The Mechanics
Cup letters aren’t a fixed size. A “C cup” on a 32 band holds less volume than a “C cup” on a 40 band, because cup volume is calculated relative to band size. When you change the band by one size (2 inches), the cup has to shift by one letter in the opposite direction to hold that same physical volume.
Why This Is Useful
If your usual size is out of stock, or a specific style runs differently, a sister size often fits nearly as well because it holds the same volume. Just distributed across a different band tension. It’s a genuinely useful backup option, rather than only a size chart trick.
Try It Yourself
Run your measurements through the Sister size calculator to see your exact sister sizes up and down.
A Worked Example With Real Numbers
Take a 34C. Its sister sizes are 32D (band down one, cup up one) and 36B (band up one, cup down one).
All three hold roughly the same cup volume, just distributed across a tighter or looser band. Someone who finds 34C bands slightly loose might actually be more comfortable in 32D without losing any cup room.
When Sister Sizing Doesn’t Work
The trick assumes the brand’s sizing is internally consistent across that jump, which isn’t always true. Some brands’ cup shapes change meaningfully between bands, rather than only volume. If a sister size feels obviously wrong in shape (rather than band tension), that’s brand-specific cut, not a flaw in the sister-size logic itself.
The Core Idea in One Sentence
A sister size holds the same cup volume as your regular size, just distributed across a different band width. Going up 2 inches in the band always pairs with dropping one cup letter, and going down 2 inches always pairs with gaining one cup letter.
Why This Isn’t a Coincidence
Cup letters are relative to band size, not an absolute measurement. A D cup on a 30-inch band is a meaningfully smaller actual volume than a D cup on a 40-inch band. Sister sizing works because both numbers in a pair describe the exact same physical volume, just measured against different band widths.
When Sister Sizing Actually Helps
It’s most useful when your usual size is sold out in a specific style, when you’re between two sizes and unsure which to try, or when switching between brands with different cut tendencies. It’s not a universal fix for every fit problem. If the band and cup both feel wrong, re-measuring is more useful than guessing at a sister size.
Why the Math Works
Cup volume is described relative to band size, not as an absolute measurement, which means the same physical breast volume can be labeled with different band-and-cup combinations depending on how tightly or loosely the band sits. Moving the band by 2 inches while shifting the cup one letter in the opposite direction keeps that same underlying volume constant, which is the entire mechanism behind sister sizing.
When Sister Sizing Doesn’t Help
Sister sizing solves a band-comfort problem while keeping the same cup volume. It doesn’t help if your actual complaint is that the cup volume itself feels wrong (too full or too empty), since by definition sister sizes hold the identical volume as your starting size. In that case, you need a genuinely different size, not a sister size.
A Concept Worth Remembering Long-Term
Understanding sister sizing once means you can apply it anytime a size is unavailable or feels slightly off, without needing to look it up freshly every time. It’s a reusable piece of sizing knowledge.
A Genuinely Useful Piece of Knowledge
Understanding this concept turns a confusing size-availability problem into a simple, solvable one. That shift in perspective alone is often the most valuable part of learning about sister sizing.
