Sister Size Bra Calculator
Find alternative bra sizes with the same cup volume. Includes comprehensive sister size charts for US, UK, EU, Australian, and French sizing systems.
Enter Your Current Bra Size
Your Sister Sizes
What Are Bra Sister Sizes?
Sister sizes are bra sizes that share the same cup volume but have different band sizes. This means the actual amount of breast tissue the cup can hold remains constant, while the band circumference changes.
For example, 34C, 32D, and 36B are all sister sizes. They each hold the same cup volume, but:
- 32D has a tighter band and larger cup letter
- 34C is the original size
- 36B has a looser band and smaller cup letter
This concept is incredibly useful when your exact size isn't available, when bras run small or large in particular brands, or when you want a slightly different fit.
Sister Size Down (Tighter Band):
Sister Size Up (Looser Band):
Cup Volume Constant:
Sister Size Charts by Region
Bra Sister Size Chart - US/UK
| ←Tighter | Original | Looser→ | ||
|---|---|---|---|---|
| 28D | 30C | 32B | 34A | 36AA |
| 28DD | 30D | 32C | 34B | 36A |
| 30DD | 32D | 34C | 36B | 38A |
| 30DDD | 32DD | 34D | 36C | 38B |
| 32DDD | 34DD | 36D | 38C | 40B |
| 34DDD | 36DD | 38D | 40C | 42B |
| 36DDD | 38DD | 40D | 42C | 44B |
| 38G | 40DDD | 42DD | 44D | 46C |
| 40G | 42DDD | 44DD | 46D | 48C |
Bra Sister Size Chart - EU (European)
| ←Tighter | Original | Looser→ | ||
|---|---|---|---|---|
| 60D | 65C | 70B | 75A | 80AA |
| 60E | 65D | 70C | 75B | 80A |
| 65E | 70D | 75C | 80B | 85A |
| 65F | 70E | 75D | 80C | 85B |
| 70F | 75E | 80D | 85C | 90B |
| 75F | 80E | 85D | 90C | 95B |
| 80G | 85F | 90E | 95D | 100C |
| 85G | 90F | 95E | 100D | 105C |
EU Band Conversion: EU band = (US band × 2.54) + 10, rounded to nearest 5
Bra Sister Size Chart - Australian
| ←Tighter | Original | Looser→ | ||
|---|---|---|---|---|
| 6D | 8C | 10B | 12A | 14AA |
| 6DD | 8D | 10C | 12B | 14A |
| 8DD | 10D | 12C | 14B | 16A |
| 8E | 10DD | 12D | 14C | 16B |
| 10E | 12DD | 14D | 16C | 18B |
| 12E | 14DD | 16D | 18C | 20B |
| 14F | 16E | 18DD | 20D | 22C |
| 16F | 18E | 20DD | 22D | 24C |
AU Band Conversion: AU band = US band − 22
Bra Sister Size Chart - French
| ←Tighter | Original | Looser→ | ||
|---|---|---|---|---|
| 75D | 80C | 85B | 90A | 95AA |
| 75E | 80D | 85C | 90B | 95A |
| 80E | 85D | 90C | 95B | 100A |
| 80F | 85E | 90D | 95C | 100B |
| 85F | 90E | 95D | 100C | 105B |
| 90F | 95E | 100D | 105C | 110B |
| 95G | 100F | 105E | 110D | 115C |
French Band: French band = EU band + 15
How to Use Sister Sizes
Know Your Starting Size
Begin with a bra size that you know fits well. This is your reference point for finding alternatives.
Identify Your Need
Determine if you want a tighter band (go down in band, up in cup) or looser band (go up in band, down in cup).
Apply the Formula
For a tighter fit: subtract 2 from band, add 1 cup letter. For looser: add 2 to band, subtract 1 cup letter.
Try It On
Sister sizes maintain cup volume but fit differently. Always try on to ensure comfort and support.
🛒 Size Unavailable
When your exact size is out of stock, try a sister size as an alternative that provides similar cup coverage.
👗 Brand Variation
Different brands fit differently. If a bra runs small in the band, try the sister size up; if large, try the sister size down.
⚖️ Weight Fluctuation
During minor weight changes, sister sizes let you maintain cup coverage while adjusting band comfort.
🎯 Between Sizes
If you're between two band sizes, sister sizes give you options to find the most comfortable fit.
Sister Size Fitting Tips
- The band provides 80-90% of support, so prioritize band fit when choosing sister sizes
- Going down a band size = tighter support; going up = looser feel
- Cup coverage should remain similar across sister sizes
- Sister sizes work best within 1-2 sizes of your true fit
- Consider adjustable straps and multiple hook positions for fine-tuning
Frequently Asked Questions
Sister sizes are bra sizes with the same cup volume but different band sizes. When you change band size, you compensate with an opposite cup size change to maintain the same breast coverage. For example, 32D, 34C, and 36B are all sister sizes.
For a tighter band: subtract 2 from your band and go up one cup (34C → 32D). For a looser band: add 2 to your band and go down one cup (34C → 36B). The cup volume stays the same.
Try sister sizes when your exact size is unavailable, when a particular brand runs small/large, during minor weight changes, or when you want slight adjustments in band tightness.
No. Sister sizes have the same cup volume but different band fits. A smaller band sister will feel tighter and provide more support, while a larger band sister will feel looser.
The sister size concept is universal, but sizing varies by brand. Always check the specific brand's size chart and consider trying on before purchasing.
Related Calculators
Official Resources
Created by OmniCalculator.space | Last Updated: January 2026