A sister size is the same cup volume on a different band. Go down a band, up a cup (34B → 32C); go up a band, down a cup (34B → 36A). The cup letter changes but the actual space inside it doesn't — because a cup letter only means anything relative to its band. This one fact explains most "the fitting room lied to me" stories.
Why a C cup isn't a fixed size
Cup letters measure the difference between your bust and your underband — not an absolute volume. A letter is a ratio, not a room. So a 32C and a 36C are entirely different sizes of body, while a 32C and a 34B hold roughly the same. When someone says "I'm a C," a fitter hears half a sentence; the band number is the half that decides whether the bra fits.
Your sister size map
| Band down (firmer) | Your size | Band up (relaxed) |
|---|---|---|
| 30C | 32B | 34A |
| 32C | 34B | 36A |
| 32D | 34C | 36B |
| 34D | 36C | 38B |
Read it as rows of equals: each row holds the same breast, on three different ribcages. One warning — sister sizing works one step in either direction. Two steps out (32C → 36A) and the geometry stops cooperating: the wires sit wrong and the straps migrate.
When to actually use a sister size
Three situations, honestly ranked. First: a style you love runs out of your size — one sister step is a legitimate substitute if the band still passes the two-finger test. Second: a brand's band runs tight or loose — European bands often run firm, so the size-up sister with the smaller cup letter restores the fit. Third — the most common — your measured size feels wrong. If the band rides up your back, the band is too big regardless of what the tape said: try the band-down sister. If the band leaves red tracks by noon, band-up sister. The letter on the label matters less than where the band sits: level, snug, parallel to the floor.
Our own bands run US-standard, 32–38, and we cut for the middle of each range — if you're between, our measuring guide takes five minutes and a soft tape, and settles it properly.
What sister sizing can't fix
Wrong cup shape. If you're spilling at the top but gaping at the sides, that's a shape mismatch — plunge vs balconette vs full cup — and no band arithmetic will solve it. Wrong wire width is the same story: wires that poke your armpit need a different cut, not a different number. Sister sizing moves a good bra one notch closer to perfect; it cannot rescue the wrong bra. (If the whole system feels stacked against you, you're not imagining it.)
FAQ
Is a sister size identical to my real size?
The cup volume is nearly identical; the band tension and wire width shift slightly with each step. One step is usually invisible in wear. Two steps rarely is.
Which sister size should I try if my band rides up?
Band down, cup up: 34B → 32C. A band that rides up is doing none of the work, and the straps are doing all of it — that's the shoulder groove you're feeling.
Do sister sizes work for bralettes?
Bralettes in S/M/L already absorb sister-size territory — each letter spans two or three band-cup combinations. Size for the underband and you've effectively sister-sized.
Why do I wear different sizes in different brands?
Band elasticity, wire width and cup depth are house decisions — a "34B" is a brand's interpretation, not a law. Sister sizing is the translation layer between those interpretations.
