Most sizing calculators show you a result and nothing else. Ours shows the work, because “trust us” isn’t good enough when it’s your money and your comfort on the line.

The Band: the “Modern” Method

We measure your underbust (snug, no padding) and round it up to the nearest even number to get your band size. This is different from the older “add 4 or 5 inches” method a lot of sizing charts still use — most fitters now consider that outdated because bands today are far more supportive than they were decades ago, so the extra padding isn’t needed. If your underbust measures 31 inches, your band is 32. If it measures 32 exactly, your band stays 32.

The Cup: Bust Minus Band

Next, we measure your bust at the fullest point (standing naturally, no padding) and subtract your underbust measurement. That difference — in whole inches — maps to a cup letter on a 13-step ladder: 0″ is AA, 1″ is A, 2″ is B, and so on up through L. A 4-inch difference on a 34-inch band, for example, works out to a 34D.

Why Every Country’s Number Looks Different

Once we know your band and cup from the two measurements above, converting to another country’s system doesn’t require new measurements — just a different label for the same physical size:

Try it yourself with the Global Size Converter — enter any one system’s size and see the other six.

Sister Sizing

Going up one band size (2 inches) while dropping one cup letter — or the reverse — keeps roughly the same cup volume, because a looser band needs a bigger cup to hold the same amount, and a tighter band needs less. That’s the whole idea behind the Sister Size Calculator.

Where This Falls Short

This is a documented, widely-used measurement method — not a medical scan, and not a substitute for trying a bra on. Brand cut, fabric stretch, and personal shape all shift how a “correct” size actually feels. Use your result as an accurate, well-reasoned starting point, then adjust from there. See our full disclaimer for more on this.