Bra Sister Sizes Explained – Sister Size Chart & Bra Size Equivalents
Bra sister sizes are bra sizes that have a similar cup volume but a different band size. When the band size changes, the cup letter changes in the opposite direction to keep the cup capacity similar.
For example, a 34F has sister sizes 32FF and 36E. These sizes follow a diagonal relationship in bra sizing charts because cup volume increases as band size increases.
Understanding bra sister sizes can make bra shopping much easier. Sister sizes are alternative bra sizes that have a similar cup volume but use a different band size.
For example, a 34F has a similar cup capacity to 32FF or 36E. The band size changes, but the overall cup volume stays roughly equivalent.
You can also explore our full UK Bra Cup Size Guide to learn how individual cup sizes progress across the sizing system.
Many bra size progressions include double cup sizes such as DD, FF and GG. These sizes simply represent additional steps in the UK cup sequence. You can learn more in our Double Cup Bra Sizes Guide.
If you are unsure of your starting size, begin with our Bra Size Calculator. You can then confirm the fit using our Bra Fitting Advice.
For a clearer idea of how these volumes appear in real life, you can also see our guide explaining how big GG cup sizes are across different band sizes.
What Are Bra Sister Sizes?
A sister size is a bra size that has the same or very similar cup volume as another size, but uses a different band measurement. This happens because cup volume increases as band size increases.
To maintain the same cup volume when adjusting the band size:
- If the band size goes down, the cup letter goes up.
- If the band size goes up, the cup letter goes down.
For example, the following sizes have similar cup volume:
- 30G
- 32FF
- 34F
- 36E
This relationship between band and cup size is why it’s important to always consider a bra size as a combination of both band and cup.
Many people assume that cup letters represent a fixed size, but cup volume actually changes with band size. Understanding sister sizing helps explain why a 34F and 36E can have very similar cup capacity.
How to Calculate Bra Sister Sizes
Bra sister sizes follow a simple rule based on the relationship between band size and cup size.
- If the band size decreases, the cup letter increases.
- If the band size increases, the cup letter decreases.
For example:
- 34F → 32FF (band down, cup up)
- 34F → 36E (band up, cup down)
This adjustment keeps the cup volume similar while changing the band measurement.
When the band size decreases, the cup letter increases.
When the band size increases, the cup letter decreases.
All of these sizes have a similar cup volume, but different band measurements.
In sister sizing, every time the band size decreases by one step, the cup letter increases by one step. If the band size increases, the cup letter decreases.
Bra sister sizes appear on a diagonal because cup volume increases as band size increases. Sizes along the diagonal have a similar cup volume even though the band size changes.
Band sizes run across the top of the chart, while cup sizes run down the side.
| 30 | 32 | 34 | 36 | 38 | |
|---|---|---|---|---|---|
| G | 30G | ||||
| FF | 32FF | ||||
| F | 34F | ||||
| E | 36E | ||||
| DD | 38DD |
The highlighted diagonal shows sister sizes. As the band size increases, the cup letter decreases to keep a similar cup volume.
For a full explanation of how cup sizes increase across the sizing system, see our Bra Cup Size Chart.
Bra Sister Size Chart (Bra Size Equivalents)
This bra sister size chart shows how bra size equivalents work when the band size changes but the cup volume remains similar. Moving one band size up or down requires adjusting the cup letter in the opposite direction to maintain a comparable cup capacity.
| Original Size | Sister Size Down (Tighter Band) | Sister Size Up (Looser Band) |
|---|---|---|
| 34F | 32FF | 36E |
| 36G | 34GG | 38FF |
| 32GG | 30H | 34G |
| 38E | 36F | 40DD |
This sister size chart helps illustrate how equivalent bra sizes align diagonally across band sizes. Although these sizes have a similar cup volume, the band measurement will still affect the overall fit and support of the bra.
Popular Bras Available in Multiple Sister Sizes
Many supportive bras are available across multiple band and cup combinations, making it easier to try different sister sizes until you find the best fit.
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Common Sister Size Adjustments
Many people search for with sister sizes when the band feels slightly too tight or too loose. Below are some common sister-size adjustments for mid-range fuller-bust sizes.
Bra Size Equivalents
Many people search for a bra size equivalent chart when a bra feels slightly too tight or too loose in the band. These equivalent sizes are usually sister sizes, meaning they maintain a similar cup volume while the band measurement changes.
For example, if a 34F feels tight around the band but the cups fit correctly, you might try the sister size 36E. If the band feels loose, the closer equivalent size may be 32FF.
These bra size equivalents follow the same diagonal relationship shown in the chart above. As the band size increases, the cup letter decreases to keep a similar cup capacity.
You can also explore how cup sizes progress across the sizing system in our Bra Cup Size Chart, or calculate your starting size using the Bra Size Calculator.
Why Sister Sizes Exist
Bra sizing is based on the relationship between band size and bust measurement. Because cup volume increases as band size increases, the same cup letter does not represent the same volume on every band size.
For example, a 32H will have less cup volume than a 38H. This is why sister sizing exists — it helps maintain similar cup capacity when adjusting the band.
To understand how cup sizes progress across the sizing system, see our UK Bra Cup Size Chart.
For general background on how bra sizing systems work, see the bra sizing reference on Wikipedia .
When Should You Try a Sister Size?
Trying a sister size can help when:
- The band feels too tight but the cups fit well.
- The band feels loose but the cup volume is correct.
- You are comparing sizes across brands with slightly different band fits.
However, sister sizing is usually best used as a temporary adjustment rather than a permanent replacement for your correct size.
Are Sister Sizes Exactly the Same?
Sister sizes have a similar cup volume, but they are not completely identical. Changing the band size also changes the overall fit of the bra.
For example, 34F and 36E have similar cup capacity, but the band on a 36E will fit more loosely around the ribcage.
This is why sister sizes are often used as a fit adjustment when the cups feel correct but the band feels too tight or too loose.
If your bra band feels too tight or too loose, it may help to double-check your starting size before trying sister sizes.
Use our Bra Size Calculator to measure your band and bust and find a recommended starting size.
Common Sister Size Questions
Here are a few common examples of sister sizes. These sizes have a similar cup volume but different band measurements.
- What is the sister size of 34F? The closest sister sizes are 32FF and 36E.
- What is the sister size of 36G? The closest sister sizes are 34GG and 38FF.
- What is the sister size of 32FF? The closest sister sizes are 30G and 34F.
- What is the sister size of 38E? The closest sister sizes are 36F and 40DD.
Explore Cup Sizes by Band Size
Sister sizing helps you adjust band size while maintaining a similar cup volume, but each band and cup combination still fits differently. You can explore how specific cup sizes behave across band sizes in our detailed guides:
F cup bras by band size, FF cup bras by band size, G cup bras by band size, GG cup bras by band size and H cup bras by band size.
These guides break down how cup volume changes with band size and help you find bras suited to your exact measurements.
Related Bra Size Guides
Find Your Correct Bra Size
If you are unsure which size to start with, measure your bust and band using our Bra Size Calculator.
You can then confirm your fit using our Bra Fitting Advice and compare sizes across brands using the Bra Size Converter.