Sister sizes, explained

You find a bra you love, your size is sold out, and someone tells you to just try a sister size. It sounds like jargon. It is actually a simple idea, and it is worth understanding, because it explains a lot about how bra sizing works.

What a sister size is

A sister size is a different band-and-cup combination that holds the same cup volume. The key thing to know is that cup letters are not absolute. A C cup is not one fixed size. It describes the difference between your band measurement and your bust, so the very same amount of breast tissue can be written more than one way depending on the band it sits on.

The rule

There are two steps, in opposite directions:

Go up a band size, and go down one cup letter.
Go down a band size, and go up one cup letter.

That keeps the cup volume the same while the band changes.

A worked example

Say you normally wear a 34C. If the 34 band feels a little tight, your sister size up is a 36B: a roomier band with the same cup. If the 34 band feels loose, your sister size down is a 32D: a firmer band with the same cup. Notice the cup letter changes even though the actual cup is the same size. That is the part that trips people up.

When a sister size helps

Your size is out of stock and you need the bra now.
The cup fits but the band is slightly tight or loose, and you want to adjust it without losing cup room.
You are between sizes and want to test which band feels better on your body.

When it does not

A sister size changes more than a number. The cup shape, the width of the wire, and where the straps sit all shift a little as you move across bands, so it is a useful workaround, not a replacement for being properly fitted. If you find yourself guessing between several sister sizes, that is usually a sign it is worth a proper fitting, where someone can look at cup shape and wire width and not just the maths. It also helps to know what a good fit looks like before you start. When you are ready, you can find a boutique near you.