Free fit tool
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.
Sister size bra calculator
Find your bra sister sizes instantly
Popular sizes — click to fill:
Your sister size chain
Detailed breakdown
All sizes below have the same cup volume. Only the band fit changes.
Remember: All sizes in the chain hold the same cup volume. Only the band tightness changes. Always try sister sizes in the same bra style so the differences are easier to compare.
What 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.
The rule is simple:
Go down one band size and up one cup letter for a firmer fit
Go up one band size and down one cup letter for a looser fit
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 |
All three sizes hold the exact same amount of breast tissue. The cup volume does not change. Only the band tightness changes. This is why professional fitters use the bra sister size calculator as a standard tool, it gives you real alternatives without compromising cup fit.
When Should You Use Sister Sizes?
The sister bra size calculator is useful in specific situations. It is a fine-tuning tool, not a replacement for finding your actual bra size.
Use your sister size when
- Your exact size is out of stock and you need a close alternative
- Your band feels slightly too tight, but the cup volume feels right
- Your band rides up at the back during the day
- A brand runs tighter or looser than the one you usually buy
- Your body has changed slightly due to weight fluctuation, hormonal shifts, or postpartum changes and you are not ready to remeasure yet
- You are switching between bra styles and the band feels different between them
Do not use sister sizing when
- Your cups overflow or spill at the sides
- Your cups gap or wrinkle at the top
- The underwire sits on breast tissue rather than flat against your ribcage
- Both the band and the cup feel uncomfortable at the same time
If any of those fit problems are happening, sister sizing will not solve them because the cup volume stays the same. In that case, use the bra size calculator to remeasure from scratch.
Sister Size Chart
Use this table as a quick reference to find sister sizes without using the bra sister size calculator. Your current size is in the middle column. The firmer band sister size is on the left, and the looser band sister size is 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 |
For sizes not shown in this table, use the sister size bra calculator above to find your exact alternatives.
Sister Sizes and Equivalent Bra Sizes
You may hear the terms "sister sizes" and "equivalent bra sizes" used interchangeably. They describe bra sizes that belong to the same size family and offer a similar cup volume while changing the band fit.
For example, these sizes are all in the same family:
These sizes are commonly considered equivalent because they offer a similar cup capacity while varying in how tightly the band fits around the body.
Sister Size vs Wrong Size
Many women confuse sister sizing with simply wearing the wrong size. The difference is important.
You May Need a Sister Size When
- The cups fit comfortably
- Breast tissue sits fully inside the cups
- The only issue is the band feeling too loose or too tight
You May Need a Different Bra Size When
- The cups overflow
- The cups wrinkle or gap
- The underwire sits on breast tissue
- Straps dig into your shoulders despite adjustments
A simple check is to see how much support the band provides on its own. If the cups fit but the band feels wrong, a sister size may help. If the cups themselves are incorrect, a different size is usually the better solution. The sister size bra calculator works best when your cup volume is already close to correct. If you are unsure, always start with a fresh measurement using the main bra size calculator first.
Sister Sizes for Different Bra Types
The sister size rule applies across all bra styles, but how much you notice the band difference varies depending on the style you are trying.
🏃 Sports Bras
Sports bras have firmer, less flexible bands. If your usual size feels too compressive during activity, try the looser band sister size, go up one band and down one cup. If your sports bra shifts around during high-impact activity, try the firmer band sister size.
✨ Strapless Bras
Strapless bras rely entirely on the band for support. If your strapless bra slides down during the day, try the firmer band sister size. A snugger band gives the bra more grip around your ribcage and reduces slipping significantly.
🌸 Bralettes
Bralettes use softer, stretchier fabric than underwired bras. Because the band has more give, the difference between sister sizes feels less pronounced. You may find that going one sister size in either direction still feels comfortable.
🔩 Underwired vs Wireless
Underwired bras have a more structured band that feels firmer at the same size. If you usually wear wireless bras and are trying an underwired style for the first time, the firmer band sister size can be a useful starting point before committing to a new size.
Sister Sizes Across Different Brands
One of the most practical uses of the bra sister size calculator is navigating sizing differences between brands. Bra sizing is not standardized globally. A 34C from one brand can fit completely differently from a 34C from another brand. This is not an error on your part, it is simply how the industry works.
Different brands grade their cups and bands using different measurements
Some brands use more stretch in their band fabric, which makes the same size feel looser
UK-based brands such as Freya, Panache, and Curvy Kate tend to follow measurement-based sizing more closely
US mass-market brands vary more, which is why trying a sister size in an unfamiliar brand is always worth doing
How to Try Sister Sizes in Store
Trying sister sizes side by side is one of the easiest ways to improve bra fit.
Use the sister bra size calculator above before you leave home. Write down your firmer and looser sister sizes for the styles you plan to try. This saves time and removes the guesswork in the fitting room.
Always compare sister sizes in the same bra style. Comparing a 34C in a padded T-shirt bra to a 36B in a balconette will not give you useful information because the styles fit differently. Same style, different sizes.
Always start on the loosest hook. This gives the band room to tighten over time as the fabric stretches with wear.
Run two fingers under the band. If they slide in easily and the band sits level around your body, the band size is right. If the band rides up at the back or you can fit more than two fingers underneath, the band is too loose. Try the firmer sister size.
Once the band feels right, look at the cups. Breast tissue should sit fully inside the cups with no spillage at the top or sides and no empty space or wrinkling. If the cups look off even though the band feels right, sister sizing is not the solution. Try a different cup size in the same band.
Only adjust the straps after you are happy with the band and cup fit. Straps should be snug enough to stay in place but loose enough for two fingers to slide under comfortably.
Common Sister Sizing Mistakes to Avoid
Sister sizing is straightforward once you understand it, but a few mistakes can lead to an uncomfortable fit.
Sister sizing only adjusts the band. If your cups do not fit correctly, going up or down a band size will not fix them. If both feel off, remeasure and start from your actual size.
Your sister sizes are one step in each direction. Going two or three steps away, for example, from 34C to 30F, makes the band difference so significant that the fit becomes uncomfortable even if the cup volume is technically the same. Stick to one step at a time.
A 34C in a T-shirt bra and a 36B in a balconette will feel different because the styles are built differently. Always compare sister sizes in the same style.
Tightening your straps when the band is too loose does not solve the problem. It just shifts the weight onto your shoulders, which causes strap marks and shoulder pain over time. Fix the band first with a firmer sister size.
The cup volume is mathematically the same. But because different brands use different fabrics, padding, and construction methods, a sister size in one brand may feel slightly different from the same size in another. Always try on rather than assuming.
Popular sister size searches
Click any size below to instantly see its full sister size chain.
Frequently Asked Questions
Not sure what size to start with?
Use our main bra size calculator to find your correct size first, then come back here to find your sister sizes.
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