Bra sister sizes are bra sizes with a similar cup volume but a different band size. If you go down one band size, go up one cup size. If you go up one band size, go down one cup size.
For example, the closest sister sizes to 34F are 32FF and 36E. You can check your own size using the Bra Sister Size Calculator or see more examples in the sister size chart below.
Bra sister sizes are alternative bra sizes with a similar cup volume but a different band size. They can be useful when you need to change the band size while keeping approximately the same cup capacity.
For example, the closest sister sizes to 34F are 32FF and 36E. Moving down a band size means moving up a cup size, while moving up a band size means moving down a cup size.
To find the closest equivalents for your own size, use our UK & International Bra Sister Size Calculator. You can also use the bra sister size chart further down this page to see how equivalent cup volumes align across different band sizes.
To compare sizes visually, try our UK Bra Size Visual Comparison Tool, which covers all 220 currently manufactured UK bra sizes. For more detail on how cup letters progress across different band sizes, see our UK Bra Cup Size Guide.
If you are unsure of your starting size, begin with our Bra Size Calculator and then check the result using our Bra Fitting Advice.
A bra sister size is a different band-and-cup combination with a similar cup volume. Cup capacity changes with band size, so the same cup letter does not represent the same volume on every band.
For example, these UK sizes have a similar cup volume:
The band measurements are different, but the cup capacities are closely related. This is why bra size should always be considered as a combination of band size and cup size, rather than by cup letter alone.
To calculate the closest sister size, change the band size by one step and adjust the cup size in the opposite direction.
For a UK 34F:
This changes the band measurement while keeping the cup volume approximately equivalent. For an instant result, use our Bra Sister Size Calculator.
Use the UK Bra Sister Size Calculator below to find the closest bra sister sizes for any UK bra size.
Use our International Bra Sister Size Calculator to find sister sizes using US, European, French, Australian and New Zealand sizing systems.
The UK sizes 32FF, 34F and 36E are sister sizes because they have a similar cup volume across different band sizes. The illustrations below show how the band and cup combination changes while the cup capacity remains approximately equivalent.
Moving from 32FF → 34F → 36E increases the band size while reducing the cup letter to maintain a similar cup volume.
Sister sizes have different band measurements, so they are not identical sizes even when their cup volumes are similar.
Bra sister sizes form a diagonal pattern in a size chart because cup volume changes with band size. Sizes along the highlighted diagonal have approximately equivalent cup volumes, even though their band measurements are different.
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 the sister-size sequence 30G → 32FF → 34F → 36E → 38DD. Each size uses a different band-and-cup combination while keeping a similar cup volume.
For a wider view of how UK cup sizes progress across different band sizes, see our Bra Cup Size Chart.
This bra sister size chart shows the closest equivalent cup volumes when moving one UK band size up or down. The band size changes by one step while the cup letter moves one step in the opposite direction.
| UK Bra Size | One Band Smaller | One Band Larger |
|---|---|---|
| 34F | 32FF | 36E |
| 36G | 34GG | 38FF |
| 32GG | 30H | 34G |
| 38E | 36F | 40DD |
The sizes in each row have a similar cup volume, but they are not identical bra sizes because the band measurement changes. Use the chart as a quick reference or enter any size into our Bra Sister Size Calculator for an instant result.
Here are some examples of how common UK bra sizes change when moving one band size smaller or larger.
For other UK sizes, use the Bra Sister Size Calculator rather than trying to calculate the cup progression manually.
Bra size equivalents are often another way of referring to bra sister sizes: different band-and-cup combinations with a similar cup volume.
For example, a UK 34F has approximately equivalent cup volume to 32FF and 36E. The band size changes, so the sizes are not identical, but the cup capacities are closely related.
If you are looking for an equivalent size because the band feels too tight or too loose, use the Bra Sister Size Calculator to check the closest sister sizes for your current size.
You can also see how cup sizes progress across different band sizes in our Bra Cup Size Chart, or check your starting size with the Bra Size Calculator.
Bra sizing is based on the relationship between band size and cup size. As the band size increases, the volume represented by the same cup letter also increases.
For example, a 32H has less cup volume than a 38H. Sister sizing adjusts both the band and cup together so that the cup capacity remains approximately equivalent.
For a wider view of how cup volume changes across different band sizes, see our UK Bra Cup Size Chart.
For another explanation of how sister sizing works, see Bravissimo’s guide to bra sister sizes .
A sister size may be useful when the cup volume feels appropriate but you need to adjust the band size.
Because a sister size changes the band measurement as well as the cup letter, always check the complete fit after changing size. Our Bra Fitting Advice explains the main band, cup and strap checks to use.
No. Bra sister sizes have a similar cup volume, but they are different bra sizes with different band measurements.
For example, 34F and 36E have a similar cup capacity, but the 36E uses a larger band measurement. The two sizes therefore should not be treated as interchangeable in every situation.
Sister sizing is best used as a way to compare equivalent cup volume when changing the band size, rather than as a replacement for checking your actual measurements and fit.
If your band feels too tight or too loose, check your starting size before moving to a sister size.
Use our Bra Size Calculator to estimate your UK bra size, then use our Bra Sister Size Calculator to compare equivalent cup volumes.
Below are some common bra sister sizes based on UK sizes people frequently search for.
For any other UK size, use our Bra Sister Size Calculator to find the closest equivalent cup volumes automatically.
Sister sizing compares similar cup volumes across different band sizes. If you want to explore how a particular cup size changes across the UK sizing system, use our cup-by-band guides:
Some bra styles are available across a wide range of band and cup combinations, making it easier to compare nearby sister sizes when adjusting the band.
Before changing size, check your starting point with the Bra Size Calculator and use the Bra Sister Size Calculator to confirm the closest equivalent cup volumes.
As an Amazon Associate I earn from qualifying purchases.
If you are unsure which size to start with, measure your bust and underband using our Bra Size Calculator.
You can then check the fit using our Bra Fitting Advice. If you need to compare UK sizing with US, European or other international sizing systems, use our Bra Size Converter.
