Sister Sizes Explained (And How to Find Yours)

You find the perfect bra. It's the right price, the right colour, and — of course — your exact size is sold out in everything but a 32DD, which is definitely not your size. Except it might be. "Sister sizes" are the one piece of bra maths worth knowing: they're why your size has back-ups, why a band that rides up has an easy fix, and why your number behaves differently from brand to brand. It's the single most useful trick in fitting.

What a sister size actually is

Here's the thing nobody tells you: a cup letter isn't a fixed amount. A D cup isn't "this many millilitres." It's a difference — roughly four inches bigger than whatever band it's attached to. So the cup on a 34D, a 32DD, and a 36C all hold about the same volume. They're sister sizes: same cup capacity, different band. The only thing that changes is how snug the band sits on your ribcage.

How to find yours

Move the band and the cup in opposite directions, one step at a time:

  • Need a tighter band? Go down a band, up a cup — a 34D becomes a 32DD.
  • Need a looser band? Go up a band, down a cup — a 34D becomes a 36C.

That's the whole rule: as the band number goes down, the cup letter goes up, and vice versa. So if you wear a 34D, your nearest sisters are a 32DD (tighter band) and a 36C (looser band) — all three with the same cup volume.

The mental unlock: stop reading the cup letter as a fixed size. It only means something next to its band number. Once that clicks, sister sizes stop being confusing and start being useful.

When to reach for a sister size

  • Your band rides up or digs in. The fix is almost always a sister-size move — tighten the band by going down a band and up a cup. (More on that in the band guides below.)
  • Your exact size is sold out. Grab a sister size instead of giving up on the bra — the cup will still fit.
  • Your "size" runs small or large in a particular brand. Shift one sister size to compensate without losing cup volume — which is half the reason your number never seems to behave across brands.

The catch

Sister sizing keeps the cup volume the same, but it isn't a free pass. A smaller band comes with a slightly narrower, shallower cup, and the band feel changes — so it's a brilliant rescue within a step or two, not something to stretch four sizes in either direction. Use it to fine-tune, not to force a fit.

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Keep reading:

What is a sister size in bras?

A sister size is a different band-and-cup combination that holds the same cup volume as your usual size. Because a cup letter is measured relative to the band, sizes like 34D, 32DD, and 36C all have roughly the same cup volume — they just grip your ribcage differently. Sister sizes let you keep the same cup capacity while changing how snug the band is.

How do I find my sister size?

Move the band and cup in opposite directions, one step at a time. For a tighter band, go down a band and up a cup: a 34D becomes a 32DD. For a looser band, go up a band and down a cup: a 34D becomes a 36C. The cup volume stays the same; only the band changes.

Do sister sizes fit exactly the same?

Not exactly. The cup volume is the same, but the band feels tighter or looser and the cup shape shifts slightly — a smaller band has a slightly narrower, shallower cup. Sister sizing is an excellent rescue for a band that's off or a size that's sold out, but it works best within a step or two; go further and the fit starts to drift.