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Bra Size Conversion: Band and Cup Across US, UK, EU, France, and Japan

A bra size is really two separate measurements β€” band and cup β€” and both are labeled differently depending on the country's sizing system.

Band size and cup size are measured independently

Band size is based on the ribcage measurement directly under the bust; cup size is based on the difference between the bust measurement and the band measurement. The two numbers combine into one size label, like 34B.

US and UK use the same band numbers but different cup scales

US and UK band sizes (32, 34, 36...) match directly, but their cup letters diverge as sizes get larger β€” a UK cup letter is commonly one step larger than the equivalent US letter once you get past a D cup, so a US DD is roughly a UK E.

EU and French sizing label the band in centimeters

Continental European sizing (and French sizing, which uses its own additional offset) expresses band size as a number derived from the underbust measurement in centimeters rather than the inch-based US/UK numbering, so a US 34 band corresponds to roughly a 75 in EU sizing.

Japanese sizing follows the EU-style centimeter band logic

Japan's band numbers are also centimeter-based like EU sizing, but the cup lettering can shift by roughly a letter compared to EU cups at the same band, so a direct letter-for-letter match between Japan and EU is not always reliable.

"Sister sizing" keeps the same cup volume across bands

Going up one band size while going down one cup letter (or vice versa) generally keeps the actual cup volume roughly constant β€” useful when your exact size is out of stock and you need the closest equivalent fit.

Why the same letter means different things in different countries

Cup letters are not an absolute size β€” they represent the difference between bust and band measurements, and each country's sizing system was standardized independently, at different times, using different reference charts. That is why a UK D and a US D do not represent the same actual cup volume once sizes get larger, even though both use the letter "D."

Conversion charts are a starting point, not a guarantee

Because bra construction (wire shape, fabric stretch, band elastic) varies significantly by brand, a converted size from a chart is best treated as a fitting-room starting point rather than a guaranteed fit, similar to how shoe and clothing size charts also vary by brand despite using the same national sizing system.

Frequently Asked Questions

What is "sister sizing" and when should I use it?

Sister sizes are different labeled sizes that hold roughly the same cup volume, found by moving one band size in one direction and one cup letter in the other. It is commonly used when your exact size isn't available or when trying a different brand's fit.

Why does a US 34B feel different from a UK 34B?

The band number matches directly, but cup lettering scales diverge between the US and UK systems as sizes increase, so at larger cup sizes a UK letter and a US letter of the same name can represent a meaningfully different actual volume.