A sister size is a nearby bra size with a different band and cup letter but a similar cup volume. It can help when the band or cup feels slightly wrong, but it is not a substitute for checking the shape and construction.
How sister sizes work
To keep a similar cup volume, go down one band size and up one cup letter, or go up one band size and down one cup letter. For example, a UK 34DD may have a similar cup volume to a 32E or 36D. Always check the sizing system used by the brand.
When to try a sister size
- The cups fit, but the band rides up: try a smaller band and a larger cup letter.
- The band feels firm, but the cups spill: try the same band with a larger cup.
- The band feels too tight, but the cups fit: try a larger band and a smaller cup letter.
When sister sizing will not help
If the wire is too narrow, the cup is too shallow or the neckline cuts in, changing numbers may not solve the problem. Try a different style as well as a different size.
Frequently asked questions
Does a sister size have exactly the same cup?
No. It has a similar volume, but the wire, depth and proportions change with the band.
Can I use sister sizes across countries?
Only after checking the brand’s conversion chart. UK, US and EU cup sequences differ.
Start with our bra measuring guide and then check the final fit.