Sister Size Bra Calculator
Select your fit issue, enter your size, and instantly find sister sizes with the same cup volume and a tighter or looser band.
Find your bra sister sizes instantly
FREEWhat Are Sister Sizes in Bras?
Sister sizes are bra sizes that share the same cup volume but use a different band size. When you change the band size by one step, the cup letter adjusts in the opposite direction to keep the volume exactly the same.
So if your size is 34C, your sister sizes are:
| Sister size | Band | Cup letter | How it feels |
|---|---|---|---|
| 32D | Smaller | Larger | Firmer around the ribcage |
| 34C | Your size | Your cup | Your current fit |
| 36B | Larger | Smaller | Looser around the ribcage |
When Should You Use Sister Sizes?
The sister bra size calculator is a fine-tuning tool, not a replacement for finding your actual bra size.
- Your exact size is out of stock
- Band feels slightly too tight but cup volume feels right
- Band rides up at the back during the day
- A brand runs tighter or looser than usual
- Your body has changed slightly
- You are switching between bra styles
- Your cups overflow or spill at the sides
- Your cups gap or wrinkle at the top
- The underwire sits on breast tissue
- Both band and cup feel uncomfortable
Sister Size Chart
Your current size is in the middle column. Firmer band on the left, looser band on the right.
| Firmer Band | Your Current Size | Looser Band |
|---|---|---|
| 30C | 32B | 34A |
| 30D | 32C | 34B |
| 30DD | 32D | 34C |
| 32B | 34A | 36AA |
| 32C | 34B | 36A |
| 32D | 34C | 36B |
| 32DD | 34D | 36C |
| 32DDD | 34DD | 36D |
| 34B | 36A | 38AA |
| 34C | 36B | 38A |
| 34D | 36C | 38B |
| 34DD | 36D | 38C |
| 34DDD | 36DD | 38D |
| 36B | 38A | 40AA |
| 36C | 38B | 40A |
| 36D | 38C | 40B |
| 36DD | 38D | 40C |
| 38C | 40B | 42A |
| 38D | 40C | 42B |
| 38DD | 40D | 42C |
| 40C | 42B | 44A |
| 40D | 42C | 44B |
| 42C | 44B | 46A |
| 42D | 44C | 46B |
Sister Sizes and Equivalent Bra Sizes
These sizes are all in the same family — they share the same cup capacity, only the band tightness changes.
Sister Size vs Wrong Size
- The cups fit comfortably
- Breast tissue sits fully inside the cups
- The only issue is the band feeling too loose or too tight
- The cups overflow
- The cups wrinkle or gap
- The underwire sits on breast tissue
- Straps dig in despite adjustments
Sister Sizes for Different Bra Types
Sports Bras
Firmer bands. If too compressive, try the looser sister size. If the bra shifts during activity, try the firmer sister size.
Strapless Bras
Rely entirely on the band. If yours slides down, try the firmer band sister size for more grip.
Bralettes
Softer, stretchier fabric. The difference between sister sizes feels less pronounced — either direction may feel comfortable.
Underwired vs Wireless
Underwired bras feel firmer. If trying underwired for the first time, the firmer band sister size is a useful starting point.
Sister Sizes Across Different Brands
Bra sizing is not standardized globally. A 34C from one brand can fit completely differently from another. Have your sister sizes ready whenever you try a new brand.
Different brands grade their cups and bands using different measurements
Some brands use more stretch in their band fabric, making the same size feel looser
UK-based brands (Freya, Panache, Curvy Kate) tend to follow measurement-based sizing more closely
US mass-market brands vary more — always have your sister sizes ready when trying a new brand
How to Try Sister Sizes in Store
Know your sister sizes before you go
Use the calculator above first. Write down your firmer and looser sister sizes.
Try the same style in both sizes
Compare sister sizes in the same bra style — different styles fit differently.
Put the bra on the loosest hook first
Start on the loosest hook — this gives the band room to tighten over time.
Check the band before anything else
Run two fingers under the band. It should sit level and snug without digging in.
Then check the cups
No spillage, no empty space. If cups look off when the band feels right, sister sizing is not the solution.
Adjust straps last
Only adjust after band and cup feel right. Straps should allow two fingers to slide under.
Common Sister Sizing Mistakes to Avoid
Sister sizing only adjusts the band. If cups do not fit, a different size is needed.
Stick to one step at a time. Two or three steps makes the fit uncomfortable.
Always compare in the same style — different styles are built differently.
Tightening straps shifts weight onto shoulders, causing marks and pain.
Cup volume is mathematically the same, but construction varies. Always try on.
Frequently Asked Questions
How Do You Calculate Sister Bra Size?
To calculate a sister bra size, go down one band size and up one cup letter, or up one band size and down one cup letter. For example, the sister sizes of 34C are 32D and 36B.
Are Sister Sizes Accurate?
Yes. Sister sizes are based on established bra sizing relationships and are commonly used by professional fitters when adjusting band fit.
Do Sister Sizes Fit Exactly the Same?
No. The cup volume remains similar, but the band fit changes. One sister size may feel firmer while another feels looser.
Can I Move More Than One Sister Size Away?
Technically yes, but moving multiple steps away usually changes the fit enough that comfort and support may suffer. Stick to one step at a time.
Can I Use Sister Sizes During Pregnancy or Postpartum?
Yes. Sister sizing can provide temporary flexibility while your body changes. Once your size stabilizes, it is best to measure again.
Do Sister Sizes Work for Sports Bras and Bralettes?
Yes. The same principles apply, although the fit difference may feel more noticeable in sports bras and less noticeable in bralettes due to differences in fabric stretch and support.
Not sure what size to start with?
Use our main Bra Size Calculator first
Find your correct size, then come back here to find your sister sizes.
Use the Bra Size Calculator →