Bra Size Calculator: How Band and Cup Size Actually Work
A bra size is really two measurements combined into one label: a band size and a cup size. Getting either one wrong is the most common reason a bra fits badly.
The band number comes from your ribcage measurement, taken snugly just under the bust. The cup letter comes from the difference between that band measurement and a looser measurement taken across the fullest part of the bust — each inch of difference moves you up one cup letter (A, B, C, and so on).
Why the Same Size Fits Differently Across Brands
Unlike shoe sizing, bra sizing has no single enforced standard. Manufacturers use different block patterns, different amounts of stretch in the band material, and different assumptions about cup shape. A 34C from one brand can measure closer to a 34B or 34D in another. This is normal, not a measuring error.
Sister Sizing
Sister sizing is the practice of moving up a band size while dropping a cup size (or vice versa) to get a similar cup volume in a different band. A 34C and a 32D hold roughly the same cup volume, just distributed across a different band. This is useful when a favorite style doesn't come in your exact size, or when you need a slightly looser or tighter band without changing how full the cup feels.
When to Remeasure
Weight changes, pregnancy, breastfeeding, and even the type of workout you do regularly can shift both measurements. A yearly remeasure, or a remeasure after any noticeable weight change of more than a few pounds, keeps the numbers current.
Frequently Asked Questions
Why do bra sizes vary between brands?
There is no single industry-wide sizing standard, so manufacturers use different patterns, stretch ratios, and cup shapes. A size that fits well in one brand may run a cup size larger or smaller in another, which is why trying a few brands at your calculated size is normal rather than a sign you measured wrong.
What is "sister sizing"?
Sister sizing means moving to an adjacent band size while adjusting the cup letter in the opposite direction to keep roughly the same cup volume — for example, a 34C and a 32D hold a similar amount of volume. It's a practical way to try a different band fit without losing the cup size that already works for you.
How often should I get remeasured?
About once a year is a reasonable default, but remeasure sooner after any noticeable weight change, after pregnancy or breastfeeding, or if a bra that used to fit well suddenly feels off in the band or cup.
What if I'm between two calculated sizes?
When you land between sizes, the band should generally take priority since it does most of the support work — try the closer band size first and use sister sizing to adjust the cup if needed.
This article is for informational purposes only. See our disclaimer.