100 bra sizes — a 12.5 cm difference is a B cup
A label for every meeting of 10 underbust measurements and 10 differences. The number is the underbust; the letter is how many steps the gap to the full bust covers. There is nothing to memorise.
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.
Two measurements, that is all
Measure the underbust straight around the body just below the breasts, and the full bust around the fullest point. Keep the tape level and snug but never tight — the fuller measurement should not compress. Those two numbers give you the label.
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.
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.
These figures are a starting point
Even at the same label, fit changes with the brand and the style, and the body itself changes over a day and over a month. The label says where to start looking; the last step is still trying it on.
Find it by underbust and difference
Underbust · The difference
Underbust 60cmBand number 60
Underbust 65cmBand number 65
Underbust 70cmBand number 70
Underbust 75cmBand number 75
Underbust 80cmBand number 80
Underbust 85cmBand number 85
Underbust 90cmBand number 90
Underbust 95cmBand number 95
Underbust 100cmBand number 100
Underbust 105cmBand number 105
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.
Frequently asked questions
Q. How do I measure for a bra?
Take the underbust and the full bust, each level around the body. The number comes from the underbust; the cup comes from the difference.
Q. What cup is a 12 cm difference?
A B. AA sits at 7.5 cm and each cup is 2.5 cm, so (12 − 7.5) ÷ 2.5 = 1.8, which rounds to two steps.
Q. What does 75B mean?
An underbust of roughly 73–77 cm and a gap to the full bust of about 12.5 cm.
Q. Is a Korean A the same as an American A?
No. The US subtracts the cup from a band measurement rather than the underbust, and sets that band larger. The letters do not map across.
Q. What if the band feels loose?
Go one band down and one cup up. When a 75B is loose, a 70C is often the closer fit.