Bra Sister Size Calculator
Find your bra sister sizes instantly. Sister sizes are alternative bra sizes with the same cup volume but different band sizes. This is useful when your usual size isn't available or if you need a slightly different fit.
Understanding Bra Sister Sizes
Sister sizes are bra sizes that share the same cup volume but have different band and cup letter combinations. When you go up in the band size, you need to go down in the cup letter (and vice versa) to maintain the same cup volume.
How Sister Sizing Works?
| Direction | Band Change | Cup Change | Example |
|---|---|---|---|
| Size Up | +2 inches | -1 letter | 32D → 34C |
| Size Down | -2 inches | +1 letter | 32D → 30DD |
When to Use Sister Sizes?
- When your usual size is unavailable in a specific style
- If the band feels too tight or too loose but the cups fit well
- When trying different brands that may run small or large
- During weight fluctuations or hormonal changes
Common Sister Size Combinations
| Smaller Band | Original Size | Larger Band |
|---|---|---|
| 28DD | 30D | 32C |
| 30DDD | 32DD | 34D |
| 32C | 34B | 36A |
| 34DD | 36D | 38C |
Important Considerations
Band Size Limits
While sister sizing works mathematically, there are practical limits:
- Going up more than one sister size may compromise support
- Bands smaller than 28 inches are rarely available in regular stores
- Very large cup sizes may have limited sister size options
Support and Comfort
Remember these key points when trying sister sizes:
- The band provides 80% of a bra's support
- Sister sizing down (smaller band) often provides better support
- Sister sizing up (larger band) may reduce support but increase comfort
Signs You Need to Try a Sister Size
Try a Smaller Band Sister Size If:
- The band rides up in the back
- You use the tightest hook on a new bra
- The straps constantly slip off
Try a Larger Band Sister Size If:
- The band leaves deep marks or feels uncomfortably tight
- You can't comfortably fasten the band
- The wire digs into your sides
Frequently Asked Questions
Can I go up or down multiple sister sizes?
While mathematically possible, it's not recommended to go more than one sister size up or down. Each step away from your true size may compromise fit and support.
Why do sister sizes exist?
Sister sizes provide flexibility in finding a comfortable fit across different brands and styles. They're especially useful when your exact size isn't available or during body changes.
How do I know which sister size direction to try?
If the band feels too tight but cups fit well, try the larger band sister size. If the band is loose but cups fit well, try the smaller band sister size.
Do sister sizes work the same for all bra styles?
Sister sizing may work differently for various styles. Bralettes and sports bras might be more forgiving, while underwire bras typically need a more exact fit.
Are sister sizes exactly the same volume?
While sister sizes have theoretically identical cup volumes, factors like brand, style, and material can affect the actual fit.
Should I buy multiple sister sizes?
It's helpful to have your true size and one sister size option, especially if you experience regular size fluctuations or shop different brands.
Do sister sizes affect bra longevity?
Wearing a too-large band (sizing up) may reduce the bra's lifespan as it will stretch faster. A properly fitted band provides optimal durability.
What about wireless bras and sister sizing?
Wireless bras are often more flexible with sister sizing since there's no underwire to consider, but proper band fit remains important for support.
Tips for Success with Sister Sizes
- Always try on bras before purchasing when possible
- Start with just one sister size up or down from your regular size
- Consider the bra style when sister sizing - some styles are more forgiving than others
- Pay attention to how the band feels, as it provides most of the support
- Remember that different brands may require different sister sizes