Quick Answer: Sister sizes have the same cup volume but different band/cup number combinations — for example, 34C and 36B hold the same cup volume. Go up a band and down a cup to size up for comfort; go down a band and up a cup to size down.
What Sister Sizing Actually Means
Cup size isn’t a fixed volume — it’s relative to band size. A C cup on a 32 band holds less volume than a C cup on a 38 band, even though both are labeled “C.” Sister sizing exploits this relationship: moving one band size up while dropping one cup letter (or vice versa) gets you a very similar cup volume in a different band.
The Sister Sizing Chart
Starting from any size, your sister sizes are found by moving diagonally:
| Smaller Band, Bigger Cup | Your Size | Bigger Band, Smaller Cup |
|---|---|---|
| 32D | 34C | 36B |
| 34DD | 36D | 38C |
| 36E | 38DD | 40D |
When to Use Sister Sizing
- Your usual size is out of stock — try a sister size before giving up on a style you like.
- A specific brand runs small or large — sister sizing gives you a starting adjustment point.
- The band feels right but the cup doesn’t, or vice versa — moving to a sister size fixes one dimension while roughly preserving the other.
What Sister Sizing Won’t Fix
Sister sizing changes the band-cup combination, not the overall support level. If your true issue is that the band itself is too loose to provide support, sizing to a tighter band in your correct cup size is usually the better fix, not a same-volume swap.
A Worked Example
Say you’re wearing a 34C but the band feels slightly loose after a few washes, while the cup fits perfectly. Rather than jumping to a full size down (34B, which would likely make the cup too small), try the sister size 32D — tighter band, same cup volume. This preserves the fit you liked in the cup while fixing the band looseness.
Sister Sizing for Discontinued or Sold-Out Styles
Sister sizing is especially useful when a beloved style is sold out in your exact size. Rather than abandoning the style entirely, checking the sister size first often gets you a genuinely comparable fit rather than settling for a completely different bra.
A Common Misconception
Sister sizing is sometimes misunderstood as meaning “these sizes are the same for everyone.” They’re not — they’re the same cup *volume*, but band circumference still changes, which affects overall support and how snug the bra feels around your ribcage. Someone who needs firm band support may still prefer their original size over a looser sister size, even with matching cup volume.
Frequently Asked Questions
Do sister sizes fit exactly the same?
Very close, but not identical — the band provides most of a bra’s support, so a looser sister-size band will feel less supportive even with the same cup volume.
Can I use sister sizing across different brands?
It works best within the same brand or style, since cup shape and band stretch vary between manufacturers.
How many sister sizes does each size have?
Typically two — one band size up with a cup size down, and one band size down with a cup size up.
