Sister sizing might be the most useful fitting concept you’ve never heard of. It’s the secret weapon that savvy bra shoppers use to find the perfect fit—even when their usual size is out of stock or doesn’t feel quite right.
What Is Sister Sizing?
Sister sizes are alternative bra sizes that have the same cup volume but different band and cup sizes.
The Rule:
- Go up a band size → go down a cup size
- Go down a band size → go up a cup size
Sister Size Chart
| Your Size | Sister Size Down (Tighter Band) | Sister Size Up (Looser Band) |
|---|---|---|
| 32A | 30B | 34AA |
| 32B | 30C | 34A |
| 32C | 30D | 34B |
| 32D | 30DD | 34C |
| 34A | 32B | 36AA |
| 34B | 32C | 36A |
| 34C | 32D | 36B |
| 34D | 32DD | 36C |
| 36A | 34B | 38AA |
| 36B | 34C | 38A |
| 36C | 34D | 38B |
| 36D | 34DD | 38C |
| 38C | 36D | 40B |
| 38D | 36DD | 40C |
| 40D | 38DD | 42C |
Example: If you wear a 34D, your sister sizes are 32DD (smaller band, larger cup) and 36C (larger band, smaller cup).
When to Try a Sister Size
Try sister sizing when:
- Your band feels too tight or too loose in your usual size
- Your cups aren’t fitting quite right
- You’re trying a different brand or style (sizing varies across brands!)
- Your go-to size is out of stock
- You have asymmetrical breasts (one slightly bigger than the other)
- You’re between sizes
Important Warning
If you have to fasten your bra on the tightest hook in your usual size, your sister size (one band size up, one cup size down) may be too big. Remember: the band—not the cups—provides the majority of the support. The band does 70–80% of the work in a well-fitted bra.
Conclusion
Sister sizing gives you flexibility when shopping. Use it wisely, and always prioritize band fit over cup letter.