Is Sister Bra Sizes Bullshit? The Truth About Sister Sizing in Bras
If you've ever gone bra shopping and been told to try a "sister size," you may have wondered: Is sister bra sizing actually legitimate, or is it just a sales tactic?
Many women are skeptical when a fitter suggests moving from a 38D to a 40C or a 36DD. At first glance, it sounds confusing. How can different bra sizes supposedly fit the same body?
The short answer is that sister bra sizes are not bullshit. Sister sizing is a real sizing method used throughout the lingerie industry. However, it is often misunderstood, misused, and sometimes relied upon too heavily when the correct bra size isn't available.
In this guide, we'll explain exactly what sister bra sizes are, how they work, when you should use them, and when you should avoid them.
What Are Sister Bra Sizes?
Sister bra sizes are different bra sizes that have approximately the same cup volume but different band measurements.
When you go up or down in the band size, the cup letter must change accordingly to maintain a similar cup volume.
For example:
| Original Size | Sister Size Up | Sister Size Down |
|---|---|---|
| 36C | 38B | 34D |
| 38D | 40C | 36DD |
| 34DD | 36D | 32DDD/F |
| 40B | 42A | 38C |
Although the band size changes, the cup volume remains relatively similar.
This is why someone wearing a 38D may find that a 36DD fits better if the band feels too loose.
Why Sister Sizing Exists
Bra Sizing Isn't Perfect
One of the biggest reasons sister sizing exists is because bra sizing is not standardized across brands.
A 38D from one manufacturer may fit differently from a 38D produced by another company.
Factors that affect bra fit include:
- Fabric stretch
- Cup shape
- Underwire design
- Strap placement
- Manufacturing differences
Because sizing varies so much, sister sizing provides flexibility when finding the best fit.
Women's Bodies Are Unique
No two bodies are identical.
Some women have:
- Wider rib cages
- Narrow shoulders
- Fuller breasts
- Shallow breast tissue
- Projected breast shapes
A sister size can sometimes solve comfort issues without dramatically changing cup capacity.
How Sister Sizing Actually Works
Many people mistakenly think a D cup is always the same size.
This isn't true.
A D cup simply means there is roughly a four-inch difference between the band measurement and the bust measurement.
As band sizes increase, cup volume also increases.
For example:
- 32D is smaller than 38D
- 38D is smaller than 42D
This is why cup letters alone are meaningless without the band size.
A 36DD and a 38D may have very similar cup volumes even though the labels look different.
When Sister Sizing Makes Sense
Your Band Feels Too Tight
If the cups fit perfectly but the band feels restrictive, you can try moving up a band size and down a cup size.
Example:
- Current size: 36D
- Sister size: 38C
This provides more room around your rib cage while maintaining similar cup capacity.
Your Band Feels Too Loose
If the cups fit but the band rides up your back, try moving down a band size and up a cup size.
Example:
- Current size: 38C
- Sister size: 36D
A snug band provides better support and helps the bra last longer.
Your Preferred Size Is Sold Out
Many women use sister sizing when their exact size is unavailable.
This can be especially useful for:
- Plus-size bras
- Strapless bras
- Sports bras
- Specialty lingerie
In many cases, the difference is barely noticeable.
When Sister Sizing Doesn't Work
Going Too Far From Your Original Size
A common mistake is using sister sizing multiple steps away.
For example:
- 38D
- 40C
- 42B
- 44A
Technically, these are connected through sister sizing.
However, by the time you reach 44A, the fit characteristics are completely different.
Most bra experts recommend staying within one sister size of your measured size.
The Cups Already Fit Poorly
Sister sizing cannot fix:
- Cup gaping
- Breast spillage
- Wire discomfort
- Wrong cup shape
These issues usually indicate that you need a completely different size or bra style.
Different Bra Styles
A plunge bra, balconette bra, minimizer bra, and sports bra may fit differently even in the same size.
Sometimes the problem isn't the size—it's the bra design.
Why Some People Think Sister Sizing Is a Scam
Retail Stores Often Use It Incorrectly
One reason sister sizing gets a bad reputation is because some stores use it to compensate for limited inventory.
For example, if your actual size is 34G but the store doesn't carry it, a salesperson may suggest a 38DD simply to make a sale.
That's not proper fitting.
Sister sizing should be used as a minor adjustment, not as a replacement for finding your correct size.
Many Women Have Been Wearing the Wrong Size
Studies and bra-fitting professionals often report that a large percentage of women wear the wrong bra size.
When someone finally discovers their true size, it may seem unbelievable.
For example:
- Wearing 40C for years
- Proper fitting reveals 36DD
Since the cup letter increased dramatically, many women assume the new size is wrong—even when it fits better.
The Biggest Myth About Bra Sizes
Cup Letters Are Not Absolute
This is perhaps the most important concept to understand.
A D cup isn't necessarily large.
Examples:
- 30D is relatively small
- 32D is moderate
- 40D is significantly larger in volume
Cup size only makes sense when paired with a band size.
This misunderstanding is responsible for much of the confusion surrounding sister sizes.
How to Know if a Sister Size Fits Correctly
Use this checklist:
The Band
The band should:
- Sit level around the body
- Feel snug but comfortable
- Stay in place during movement
The Cups
The cups should:
- Fully contain breast tissue
- Show no spilling
- Have no empty gaps
The Underwire
The underwire should:
- Sit against the rib cage
- Not rest on breast tissue
- Follow the natural breast root
The Center Gore
The center panel between the cups should lie flat against the chest.
If it lifts away, the size may not be correct.
Should You Use Sister Sizes?
Yes, But Use Them Properly
Sister sizing is a legitimate bra-fitting technique used by professional fitters and lingerie brands worldwide.
It can help when:
- A band is slightly too tight
- A band is slightly too loose
- Your size is unavailable
- Different brands fit differently
However, sister sizing is not a magic solution.
If a bra is dramatically uncomfortable or clearly the wrong size, moving through multiple sister sizes won't solve the problem.
Final Verdict: Is Sister Bra Sizing Bullshit?
No, sister bra sizing is not bullshit.
The concept is based on real bra-sizing mathematics and has been used throughout the lingerie industry for decades. The confusion comes from the fact that many retailers misuse sister sizing, and many shoppers misunderstand how bra sizing actually works.
When used correctly, sister sizes can help you find a more comfortable fit, especially when dealing with different brands, fabrics, and bra styles. The key is to use sister sizing as a small adjustment tool—not as a substitute for knowing your true bra size.
If you've struggled with uncomfortable bras, learning how sister sizing works could be the missing piece that helps you finally achieve a supportive, comfortable fit.