Sister Sizes Explained: How to Find Your Fit When Your Size Is Sold Out
Your size is out of stock, but the bra you love comes in two neighbors that fit almost identically. Sister sizing is the single most useful trick in bra shopping.
Sister sizes are bra sizes that share the same cup volume but have different band measurements.
Because the cup is always relative to the band, you can move up or down a band size and adjust the cup letter to keep the same actual cup capacity.
The result is a different size on the label that fits your body almost the same way.
The rule
To find a sister size:
- Go down a band size, go up a cup letter. A 34C becomes a 32D.
- Go up a band size, go down a cup letter. A 34C becomes a 36B.
All three — 32D, 34C, 36B — have the same cup volume. They differ only in how tightly the band wraps your ribcage.
Why this is so useful
Three reasons it earns its place in your shopping toolkit:
- Your size is sold out. Slip into the sister size and you keep the same cup with a slightly different band feel.
- The band is the problem, not the cup. If your usual size fits in the cup but the band rides up, drop to the sister size with the tighter band — e.g. 36C to 34D.
- Brands run small or large. When a brand's band runs tight, jump to the looser sister size instead of giving up on the bra.
A worked example
Say you normally wear a 34D, the band feels loose by mid-afternoon, and it rides up when you reach for something. The cup is fine. Move to the sister size with a firmer band: 32DD.
Same cup volume, tighter band, no more riding up.
Where it has limits
Sister sizing is a clever workaround, not a cure-all. Move more than one step in either direction and the band starts to feel genuinely wrong, even if the cup volume technically matches.
Straps and wire shapes are also designed around the labeled size, so a two- or three-step jump can leave straps too long or wires the wrong width.
Stay within one step for the most reliable results, and confirm the fit against our fit checklist.
A quick reference
If your size is 34C, your closest sisters are 32D (tighter band) and 36B (looser band). Apply the same one-step logic to any size you wear — once it clicks, you will never be fully stuck by an "out of stock" again.