Why So Many Women Wear The Wrong Size
Studies on bra fit consistently find that most women are wearing a band that's too large and a cup that's too small — often without realizing it, because that combination doesn't always feel obviously wrong day to day. The band, not the straps, should be doing about 80% of the support work; if your straps dig in and leave marks, that's usually a sign the band is too loose, not that you need smaller cups.
How This Calculator Works
This uses the traditional "add 4 or 5" method: your rounded underbust measurement is adjusted up to the nearest even number most brands actually manufacture, then the gap between that band size and your bust measurement determines your cup — each inch of difference is roughly one cup size. It's the same starting formula most lingerie fitters begin with before adjusting for the specific bra on your body.
Understanding Sister Sizes
If a size doesn't feel right, its "sister size" is usually the next thing worth trying: going down a band size and up a cup size (or the reverse) keeps the cup volume roughly the same while changing how the band fits. A 34C and a 36B hold approximately the same cup volume — just distributed across a different band.
Signs Your Current Bra Doesn't Fit
- The band rides up in the back instead of sitting level all the way around.
- You need the loosest hook when the bra is new (leaving no room to tighten as it stretches with wear).
- There's spillage over or gapping under the cup.
- The center point between the cups doesn't sit flat against your sternum.