Cup Sizes Explained — How Band, Bust & Cup Work Together

Cup size is the difference between your bust measurement and your band measurement, converted to a letter — 1 inch is AA, 2 inches is A, 3 inches is B, and so on. It only means anything alongside your band number, since the same cup letter fits differently on different bands.

Cup Size Basics

To calculate cup size, subtract your band measurement from your bust measurement — the result in inches maps directly to a letter. Most UK charts list this in inches by default, since that's the traditional unit for UK bra sizing, though the maths works identically in centimetres.

Why the Same Letter Means Different Things on Different Bands

Cup size isn't a fixed measurement on its own — it's entirely relative to band. A D cup on a 32 band holds less actual volume than a D cup on a 38 band, even though the letter is identical. The same one-inch gap means a smaller cup on a larger band and a bigger cup on a smaller band — which is exactly why sister sizes exist.

Band Size Basics

Your band size is your underbust measurement, rounded to the nearest even number, and it's the foundation the cup calculation sits on top of. Underbust is measured directly beneath your bust, on your ribcage — snug enough that the tape doesn't slide, loose enough for two fingers underneath.

Bust, Breast and Boob Measurement — Same Thing, Different Words

Whatever word feels natural to you — bust, breast, or boob — the method is identical: find the fullest point of your chest with the tape level, without a padded bra. None of these words change the underlying maths, they're just different ways of describing the same reading.

For the complete step-by-step measuring technique, see our guide on how to measure your bra size at home. Once you have both numbers, use our bra cup size calculator to get your result instantly, or check the full UK bra size chart to find your row directly.

Frequently Asked Questions

Can my cup size change without my band changing?

Yes — weight changes, hormonal shifts, and even the time of month can affect bust measurement more than band measurement, so it's common for cup size to shift while band size stays the same.

Why do I seem to be a different cup size in every brand?

Every brand cuts its cups to a different shape and depth, so the same letter can look and fit differently between brands. Treat your calculated letter as a reliable starting point, then adjust by one cup size when trying somewhere new.

Is there a maximum cup size?

Not really — mainstream UK retailers commonly stock up to G or GG, but specialist and made-to-measure brands go considerably further, into K and beyond, for anyone mainstream stock doesn't cover.

What's the difference between AA and A?

AA means almost no difference between your bust and band measurement, while A means roughly a one-inch difference. Both are common, especially on smaller bands, and neither is unusual.

Why does the same cup letter feel bigger on a smaller band?

Because cup volume is proportional to band size — a D cup on a 32 band holds less actual volume than a D cup on a 38 band, even though the letter is identical. This is exactly why sister sizes exist.