90 cm underbust, 12.5 cm difference — 90B

Each cup is 2.5 cm apart

In Korea and Japan the gap between full bust and underbust is called AA at 7.5 cm, and every further 2.5 cm moves up one cup: 10 cm is A, 12.5 cm is B, 15 cm is C. So a measured gap of 12 cm gives (12 − 7.5) ÷ 2.5 = 1.8, which rounds to two steps — a B. No table needed.

Change the band and the cup follows

Move the band up one step and the same letter is a bigger cup, because the cup is defined by a difference and a wider band spreads that difference over more fabric. So when a 75B is loose, the closer match is often 70C rather than 80A — the pairs shops call sister sizes.

Nearby cells

Same underbust, other differences

Same difference, other underbusts

The same letter means different things

American and British labels subtract the cup from a **band** measurement, not from the underbust, and that band is set larger than the underbust. Because the two systems measure from different places, the letters do not map onto each other — an A here is not an A there. So this table shows the two raw measurements instead of translating letters. When buying from an overseas brand, read that brand’s own centimetre chart.

How to read this

• The number in the label is the underbust read to the nearest 5 cm.
• The cup comes from full bust minus underbust — 7.5 cm is AA, and every 2.5 cm is one step.
• US and UK labels measure from a different place, so the letters do not translate directly.
• Brand and style change the fit. The label is where you start.