Bra Size Calculator
Your bra size from two measurements, in UK, US, EU and French sizes.
Good to know. A cup letter means nothing on its own: a 32D and a 38D hold very different amounts.
Approximate. Real-world values vary.
How it works
In plain words
The band is your underbust rounded to the nearest 5 cm. The cup comes from how much larger the bust is: 12–14 cm is an A, and each 2 cm more is one cup up. UK and US bands are in inches (EU 75 is 34); French bands are 15 more than European ones.
The formula
EU band = underbust rounded to 5 cm cup step = ⌊(bust − underbust − 10 cm) ÷ 2 cm⌋ (AA, A, B, C, D, E…) UK band = 34 + (EU band − 75) ÷ 5 × 2 FR band = EU band + 15
Worked example
An underbust of 75 cm and a bust of 91 cm are 16 cm apart, so the size is 75C in Europe, 34C in the UK and US, and 90C in France. The sister sizes, 80B and 70D, hold the same cup volume.
Sources
No outside data or rates: this is plain arithmetic.
4 automated tests check its answers on every change.
About the bra size calculator
A bra size is two things: the band, which goes around the ribcage and does most of the supporting, and the cup, which depends on how much larger the bust is than the band. This calculator finds both from two measurements and writes the result in the main sizing systems.
It uses the European standard, which measures directly in centimetres: the band is the underbust rounded to 5 cm, and each 2 cm of difference is one cup. UK and US sizes are converted with the usual tables. Some UK and US shops use other methods, such as adding inches to the underbust, which is why the same person can be told different sizes.
Sister sizes hold the same cup volume on a different band: one band up and one cup down, or one band down and one cup up. If a band rides up at the back, go down a band and up a cup; if it digs in, the reverse.
Treat the answer as a starting point. Sizes differ between brands and styles, and how a bra fits matters more than the label: a firm band level all the way round, no gaps or spilling in the cups, and the centre lying flat. A fitting in a shop is worth it. For other clothes, use the clothing size converter.
Questions people ask
How do I measure my bra size?
Measure snugly around your ribcage just under the bust, then loosely around the fullest part. The difference gives the cup: 12–14 cm is an A, and each 2 cm more is one size up.
What is 75C in UK and US sizes?
34C in both. In France it is 90C.
What are sister sizes?
Sizes with the same cup volume on a different band, like 80B, 75C and 70D.