Cup Size Measurement, Demystified

A cup size is a subtraction: your bust at its fullest minus your band, with each inch of difference earning one letter. It is not an absolute volume, which is why a 30D is small and a 40D is not, and why the same breasts wear different letters on different bands.

Measuring the difference right

Both numbers come from the two-step measurement in our measuring guide: snug underbust, relaxed fullest-point bust, tape level for both. The subtraction is only as good as its inputs, and the classic errors, padded-bra bust readings and drooping back tape, both inflate the letter. Soft tissue earns the lean-forward refinement: measure the bust leaning 90 degrees and average with standing.

The letter ladders, system by system

Up to D, everyone agrees: one inch A, two B, three C, four D. At five inches the family argument starts. US and UK both say DD, then US stacks DDD where UK says E, and from there UK inserts doubled letters (FF, GG, HH) that US and EU ladders skip. Japan and Korea run their letters about one fuller throughout. The consequence: cross-system shopping by letter alone drifts a size per border, which is what the converter exists to fix.

When the calculated letter feels wrong

The calculation gives a starting letter; construction decides how it wears. Shallow-cut brands make your true letter spill, deep-cut brands make it gape, and both feel like the math failed when the pattern just disagrees with your shape. Verify with the 10 fit checks and adjust by symptom, not by doubting the tape.