Bra Sister Sizes Australia: What They Mean and When to Use Them

Sister sizing is one of the most useful bra concepts once you understand it, and one of the most abused once people half understand it. Used properly, it helps you adapt when a band is a bit tight, a brand runs weird, or your exact size is out of stock. Used badly, it becomes a long-term excuse for wearing the wrong size and pretending it is strategic.

The core idea is simple. Cup volume changes when band size changes, so if you go up a band size you usually go down a cup size to keep a similar cup volume. And if you go down a band, you go up a cup.

How sister sizing works

If the band goes up one step, the cup goes down one step. If the band goes down one step, the cup goes up one step. That is the basic rule.

• 14D ↔ 12DD ↔ 16C
• 14E ↔ 12F ↔ 16DD
• 16E ↔ 14F ↔ 18DD
• 10E ↔ 8F ↔ 12DD

When sister sizes are useful

• Your exact size is sold out
• A specific brand runs tight in the band
• A specific brand runs loose in the band
• The cup volume feels right but the band fit is slightly off

This is where sister sizes shine. They are a tactical adjustment, not a random guess.

When sister sizes are not the answer

If the wires sit on tissue, the cup shape is wrong, the centre gore never sits well, or the whole bra feels wrong in multiple ways, sister sizing may not fix it. At that point the issue could be shape, brand cut, or your base size being off.

Put simply, sister sizes are helpful, but they are not magic.

FAQ