Bra sister size calculator
Same cup volume, different band. Tap any size in the grid to see its diagonal of sisters.
The sister-size grid
Every row is a band, every column a UK cup. The yellow diagonal through your size holds roughly the same cup volume. Tap a cell to jump to it.
| Band | AA | A | B | C | D | DD | E | F | FF | G | GG | H |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 28 | ||||||||||||
| 30 | ||||||||||||
| 32 | ||||||||||||
| 34 | ||||||||||||
| 36 | ||||||||||||
| 38 | ||||||||||||
| 40 | ||||||||||||
| 42 | ||||||||||||
| 44 | ||||||||||||
| 46 |
Why it works: a cup letter is relative to its band. A D cup on a 38 band holds more than a D cup on a 30 band, because the whole cup is scaled to a wider ribcage. Each band step (2 in) scales the cups up by roughly one letter's worth of volume, so dropping one letter as you go up one band keeps the underwire width and cup volume about the same.
Questions shoppers ask
What is a sister size?
A sister size keeps the same cup volume on a different band. Going one band size up means going one cup letter down (34C → 36B), and one band down means one cup up (34C → 32D).
What is the sister size of 34B?
34B sister sizes are 32C (tighter band) and 36A (looser band). Further out: 30D and 38AA.
What is the sister size of 32DD?
32DD sister sizes are 34D (looser band) and 30E (tighter band) in UK sizing.
When should I use a sister size?
Use the band-down sister if the band rides up or you are on the tightest hook; use the band-up sister if the band digs in but the cups fit. If the cups gape or spill, the cup letter itself is wrong, not the band.