Have you ever found the most beautiful bra, only to discover your exact size is sold out? Don't panic. You might be able to buy it anyway by using a sister size.

The Golden Rule of Bra Sizing

A "C cup" is not a static size. The volume of a C cup on a 32 band is much smaller than the volume of a C cup on a 38 band. The cup letter only represents the ratio between your bust and your ribcage.

How to Find Your Sister Size

Sister sizes share the exact same cup volume, but have different band lengths. To find yours, remember this simple rule:

  • Go up a band size, go down a cup size.
  • Go down a band size, go up a cup size.

For example, if you normally wear a 34C, your sister sizes are:

  • 36B (Up a band, down a cup)
  • 32D (Down a band, up a cup)
Both a 36B and a 32D have the exact same breast volume capacity as a 34C!

When Should You Use Them?

Sister sizes are perfect when dealing with different brands. Lingerie brands from the UK might run tighter in the band than brands from the US. If a 34C feels too tight around your ribs but the cups fit perfectly, you should try a 36B.

If you're buying internationally, you'll also want to make sure the sizing translates correctly. Use our Size Converter to ensure you're getting the right equivalent!

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