Sister Size Bra Calculator
A sister size is a different band and cup combination that holds the same cup volume as your current size. Going down a band means going up a cup letter to keep the volume; going up a band means going down a letter. Enter your size and the tool lists the whole family.
Sister sizing is how you fix a bra that fits in the cup but not the band, and how you shop brands whose bands run firm or stretchy.
Move one step when a bra fits in the cup but not the band: band too loose, go down a band and up a cup letter. Band too tight, the reverse. The printed letter changes but the cup volume stays the same.
Runs entirely in your browser. Nothing you enter is stored or sent anywhere.
Why a 32DD, 34D and 36C hold the same cups
Cup letters are not absolute volumes; they describe the difference between bust and band. A DD on a 32 band means five inches over 32, while a C on a 36 means three inches over 36, and those work out to nearly identical cup capacity. The letter grows as the band shrinks because the same tissue projects further from a smaller frame.
This is also why 'I am a D cup' means little without the band number. A 30D holds materially less than a 38D. Whenever you compare sizes or shop reviews, carry both numbers together.
When to move along the sister ladder
Move one step down in band (and up in letter) when the cups fit but the band rides up, slides or spins: the smaller band anchors while the raised letter preserves your cup room. Move the other way when a band digs painfully on the loosest hook from day one. Move at most one step at a time; two steps changes wire width and cup shape enough to need a full refit.
Frequently asked questions
Last updated August 2026.