Bra Size Calculators

UK & International Bra Sister Size Calculator

Use our Bra Sister Size Calculators to find bra sizes with a similar cup volume on a smaller or larger band. Choose the UK calculator for standard UK bra sizing, or use the international calculator to work with UK, US, European, French, Australian and New Zealand size labels.

Sister sizing changes the band size and cup letter together:

  • Choose a smaller band and move up one cup step.
  • Choose a larger band and move down one cup step.

For example, the closest UK sister sizes to 34F are 32FF and 36E. These sizes are designed to provide a similar cup capacity, but the band length and overall bra proportions are different.

For a complete explanation of how this relationship works, visit our Bra Sister Sizes guide.

Before using a sister size

If you are unsure of your current size, enter your underbust and fullest bust measurements into our Bra Size Calculator. You can then check the band, cups, wires and straps using our Bra Fitting Advice.

UK Bra Sister Size Calculator

Select your current UK bra size. The calculator will show:

  • the closest sister size on a smaller band;
  • your selected size;
  • the closest sister size on a larger band.

The calculator follows the UK cup sequence, including double letters such as DD, FF, GG, HH, JJ, KK and LL.

UK example: 34F
  • 32FF uses a smaller band and a cup one UK step higher.
  • 34F is the selected starting size.
  • 36E uses a larger band and a cup one UK step lower.

These sizes have a similar cup volume, but they are not identical sizes. The band length, wire position, strap placement and overall bra proportions can change.

International Bra Sister Size Calculator

Select a sizing system and bra size to find the closest sister sizes using the cup progression for that system.

International cup labels do not always follow the same sequence. For example, UK sizing commonly uses double letters such as FF and GG, while other systems may use labels such as DDD, F or G at similar points in their cup progression.

Why does the calculator sometimes show two cup letters?

International cup labels can vary between manufacturers. In US sizing, for example, the same point in the cup sequence may be labelled DDD by one brand and F by another. The calculator shows common alternatives separated by a slash, such as DDD/F.

Always check the size chart supplied by the individual brand before ordering.

How Bra Sister Sizing Works

A bra size combines a band size with a cup size. Cup letters do not represent one fixed cup volume across every band size.

As the band size increases, the volume represented by the same cup letter also increases. Sister sizing adjusts the cup letter in the opposite direction when the band changes.

Adjustment Band Change Cup Change 34F Example
Sister size down One band smaller One cup step higher 32FF
Current size No change No change 34F
Sister size up One band larger One cup step lower 36E

You can see a fuller explanation, additional diagrams and a UK sister-size chart in our Bra Sister Sizes Explained guide.

For general background on how bra sizing systems work, see the bra size overview on Wikipedia .

Bra Sister Size Examples

UK 32FF

Closest UK sister sizes: 30G and 34F.

UK 36G

Closest UK sister sizes: 34GG and 38FF.

UK 38E

Closest UK sister sizes: 36F and 40DD.

Bra Sister Sizing vs International Size Conversion

These two tools answer different questions:

  • Sister-size calculator: “Which smaller-band or larger-band size has a similar cup capacity?”
  • International size converter: “What is this same bra size called in another country?”

For example, changing from 34F to 36E is sister sizing because both the band and cup have changed. Converting a UK size into its US or European label is international size conversion.

Need to convert a size between countries?

Use the International Bra Size Converter on our Bra Size Guide page.

To compare cup-size progressions used by different lingerie brands, use our Bra Size Converter.

When Might You Try a Sister Size?

A sister size may be worth checking when the cup volume appears suitable but the band needs adjusting.

  • The cups fit, but the band feels too tight.
  • The cups fit, but the band feels too loose.
  • Your usual band-and-cup combination is unavailable in a particular range.
  • You want to compare a nearby size in a different bra style.

Sister sizes are related by cup volume, but they are not interchangeable in every bra. Changing the band changes the foundation of the fit, so the complete bra should always be checked.

Check the Fit of a Sister Size

  • Band: It should sit level around the body and remain in position.
  • Cups: Breast tissue should be contained without cutting in, spilling or empty space.
  • Centre gore: In most wired bras, the centre should sit against the chest.
  • Underwires: The wires should sit around the breast tissue rather than on it.
  • Straps: They should assist with positioning without carrying most of the support.

Follow the complete checks in our Bra Fitting Advice before deciding whether the sister size is suitable.

See sister sizes side by side

Use our UK Bra Size Visual Comparison Tool to compare sister sizes using original torso illustrations and bust measurements.

Related Bra Size Tools and Guides

Bra Sister Size Calculator FAQs

Q: What is a bra sister size?
A: A bra sister size is a band-and-cup combination with a similar cup volume to another size. The band changes while the cup letter moves in the opposite direction.

Q: What are the sister sizes of UK 34F?
A: The closest UK sister sizes are 32FF on a smaller band and 36E on a larger band.

Q: Are sister sizes exactly the same size?
A: No. They have a similar cup volume, but the band length and overall bra proportions are different.

Q: Can I move more than one band size?
A: Sister-size relationships can continue across several band sizes, but the overall fit changes further with every step. The closest sister size on either side is usually the clearest starting point for comparison.

Q: Does the international calculator convert my UK size into a US or European size?
A: No. It finds sister sizes within the selected sizing system. To convert one size label between countries, use the International Bra Size Converter.

Q: What should I do if I do not know my starting size?
A: Measure your underbust and fullest bust and enter the measurements into the Bra Size Calculator. Then use our Bra Fitting Advice to check the result.

Learn more about sister sizing

Read our complete Bra Sister Sizes Explained guide for visual examples, a sister-size chart and more detailed fitting guidance.