Same Volume.
A Different Fit.
A sister size has the same cup volume as your current size, just on a different band perfect for when your usual size is out of stock, or your band feels slightly off but the cup feels right. Pick your current size below to find your sister sizes instantly.
- ✓ Results in under a minute, no sign-up needed
- ✓ Covers AA through J cup, band 28–48
- ✓ Shows sister size up and sister size down
Find your sister sizes
Select your current band and cup size — the size you already know fits your cup volume.
How Sister Sizing Works
Every time you move one band size up, the cup needs to hold the same volume across a bigger band so the cup letter drops by one to compensate. The reverse happens moving down: a smaller band means the cup letter goes up by one to keep the same volume. That's the entire rule: band up, cup down band down, cup up. Sister sizes won't fit exactly like your original size (the band still changes how supportive it feels), but the cup volume stays the same, which makes them a reliable backup when your size is unavailable.
Step-by-Step: How to Find Your Sister Size
- Start with a size you already know fits your cup volume well, even if the band feels slightly loose or tight.
- Select that band and cup in the calculator above.
- Check "Sister Down" if you want a firmer band with the same cup volume in a bigger-lettered cup.
- Check "Sister Up" if you want a looser band with the same cup volume in a smaller-lettered cup.
Sister Size Chart (Examples)
| Sister Down | Your Size | Sister Up |
|---|---|---|
| 32D | 34C | 36B |
| 34D | 36C | 38B |
| 30E | 32DD | 34D |
| 36DD | 38D | 40C |
Frequently Asked Questions
Will a sister size fit exactly the same as my usual size?
No the cup volume matches, but the band tightness and strap angle will feel different. Use it as a close backup, not an identical replacement.
Why would I need a sister size?
Common reasons: your usual size is sold out, a specific style only comes in certain bands, or your band fits but the cup gapes or spills slightly.
Can I go more than one sister size away?
Technically yes, but volume accuracy drops the further you move one step in either direction is the most reliable.