Sister Sizes Explained
Sister sizes have the same cup volume but different band sizes. They're the secret weapon for finding bras that actually fit comfortably.
The Core Concept
When you go up a band size, you go down a cup size to keep the same cup volume. And vice versa.
All three sizes have the exact same cup volume.
Interactive Sister Size Tool
Sister Sizes: The Secret to Perfect Fit
Same cup volume, different band sizes. Game-changer for finding comfort.
Your size: 34D
Band feels too loose?
Try a sister size down - smaller band, larger cup. The band will be snugger, but cup volume stays the same.
Band feels too tight?
Try a sister size up - larger band, smaller cup. More breathing room, same cup volume.
Pro Tips
- • Band provides 80% of support - it should be snug but comfortable
- • You should be able to fit two fingers under the band
- • Cups that gap? Try sister size down. Spillage? Try sister size up.
- • Different brands fit differently - sister sizes help you find the right one
When to Try a Sister Size
Try a Sister Size DOWN when:
- • Your band rides up your back
- • You're on the tightest hook from day one
- • The band feels loose even when new
- • You need more support
Example: 34D → 32DD
Try a Sister Size UP when:
- • Your band feels too tight
- • The band digs in or leaves marks
- • You can't comfortably fit two fingers under the band
- • You want more breathing room
Example: 34D → 36C
Sister Size Chart
| Sister Size Down | Your Size | Sister Size Up |
|---|---|---|
| 30DD | 32D | 34C |
| 32D | 34C | 36B |
| 34E | 36DD | 38D |
| 36C | 38B | 40A |
| 38DD | 40D | 42C |
Pro Tips
Things to remember about sister sizes:
- •The band provides 80% of support. A well-fitting band is crucial - sister sizes help you find it.
- •The two-finger test: You should be able to fit two fingers under your band. No more, no less.
- •Cup gapping? Before going down a cup size, try a sister size down. The tighter band may solve the gap.
- •Cup spillage? Before going up a cup, try a sister size down. Sometimes a too-loose band causes spillage.
- •Different brands fit differently. You might be 34D in one brand and 32DD in another.
Related Guides
Frequently Asked Questions
What are sister sizes in bras?
Sister sizes are bra sizes that share the same cup volume but have different band sizes. For example, 32D, 34C, and 36B are all sister sizes because the cup holds the same amount of breast tissue, even though the band measurement differs. This relationship exists because cup size is relative to band size, not an absolute measurement.
When should I try a sister size?
Try a sister size when your current bra's band feels too tight or too loose but the cups fit well. If the band digs in or leaves red marks, go up one band size and down one cup size (e.g., 34D to 36C). If the band rides up your back or feels loose, go down one band size and up one cup (e.g., 34D to 32DD).
Does the cup volume really stay the same between sister sizes?
Yes, the physical volume of the cup is identical across sister sizes. A 32DD cup holds the exact same amount as a 34D cup or a 36C cup. The difference is entirely in the band length and the wire width. However, the shape of the cup may feel slightly different because wider bands spread the same volume over a wider frame.
Can I go more than one sister size up or down?
While it is technically possible to move two steps in either direction, most fitting experts recommend staying within one sister size of your measured size. Going two sizes away (e.g., 32D to 36B) can significantly change the fit because the wire width, strap placement, and overall bra geometry differ too much for a comfortable fit.
Do all bra brands support sister sizing?
The sister size concept applies universally across all brands because it is based on the mathematical relationship between band and cup measurements. However, brands fit differently due to variations in materials, cut, and construction. You may be your standard size in one brand and a sister size in another, which is why trying on is always recommended.
Find Your Perfect Sister Size
Our AI considers sister sizes automatically when recommending products for you.