If you've ever been told to "just try a sister size," but weren't quite sure what that meant, this is the short version — plus a video that walks through it visually.
Video: “How Do Bra Sizes Actually Work? What Are Sister Sizes?” · hosted on YouTube.
The one rule to remember
Sister sizes exist because cup volume is relative to band size — the same cup letter holds more on a bigger band than a smaller one. So moving diagonally across a size chart, up a band and down a cup (or the reverse), lands you on a size with roughly the same cup volume.
band down 1 step, cup up 1 letter = same volume, tighter band
A 34C, for example, shares roughly the same cup volume as a 32D and a 36B — three different labels, one similar fit around the cup.
When to use it
Good reasons to try a sister size
Your exact size isn't available
A sister size often gives a very similar fit when the original isn't in stock.
Cup fits, band doesn't
Sizing down the band and up the cup keeps cup volume while fixing a loose band.
A brand runs differently
Trying a sister size is a quick way to compensate for a brand that runs small or large.
Questions
Sister sizing FAQs
Cup volume is similar, but band tightness and overall shape differ, so it's a close alternative rather than an identical fit.
Yes, the same diagonal logic extends further, though fit becomes less precise the further you move from your true size.
Usually because they don't stock every size, so a sister size fills the gap with a comparable fit.
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