Last updated: August 2026
Sister sizes are bra sizes with the same cup volume but a different band. They are the single most useful trick in bra fitting: when the band feels wrong but the cups are right, the sister size fixes it without starting over.
What a sister size is
A cup letter is a difference between your bust and your band — not a fixed volume. So if you keep the cup volume the same and change the band, the letter has to change with it. Those matched sizes are sisters. A 34C, for example, holds the same cup as 32D and 36B.
How sister sizes work
- Band too tight? Go up one band and down one cup — 34C becomes 36B. Same cup volume, roomier ribcage.
- Band too loose or riding up? Go down one band and up one cup — 34C becomes 32D. Firmer, more supportive band, same cups.
Sister size chart
Find your size, then try a sister when the band feels off:
| Your size | Snugger band (cup up) | Looser band (cup down) |
|---|---|---|
| 30A | 28B | 32AA |
| 30B | 28C | 32A |
| 32B | 30C | 34A |
| 32C | 30D | 34B |
| 32D | 30DD | 34C |
| 34B | 32C | 36A |
| 34C | 32D | 36B |
| 34D | 32DD | 36C |
| 34DD | 32DDD | 36D |
| 36C | 34D | 38B |
| 36D | 34DD | 38C |
| 36DD | 34DDD | 38D |
| 36DDD | 34G | 38DD |
| 38D | 36DD | 40C |
| 38DD | 36DDD | 40D |
| 40DD | 38DDD | 42D |
How to check the fit is actually right
Numbers get you to the right size on paper; these hands-on checks tell you whether it fits in real life. Do them in front of a mirror, wearing the bra on the loosest hook:
- Band: it should sit level all the way around, in line with the front. Slide two fingers under it at the back — snug, not loose, no riding up when you raise your arms. The band does roughly 80% of the support work, so this matters most.
- Cups: do a quick “scoop and swoop” — lean forward, settle everything into the cup, then stand up. There should be no spillage over the top or sides, and no wrinkling or gaping at the edge. Spillage means size up a cup; gaping means size down a cup.
- Centre gore: the little bridge between the cups should lie flat against your breastbone. If it floats off the body, the cup is usually too small.
- Straps: you should fit a finger under them comfortably. If they dig in or carry most of the weight, the band is too loose — tighten it or size the band down.
A worked example
Say you wear 36C but the band rides up and the straps dig in — classic signs the band is too loose. Drop a band and add a cup: 34D. The cups hold the same amount, but the firmer band sits level and takes the weight off your shoulders. If instead a 36C band left red marks and felt breath-restricting, you would go the other way to 38B.