How to measure
Measure your band
Wrap a soft tape snugly around your ribcage, directly under your bust. This is your underbust measurement.
Measure your bust
Measure loosely around the fullest part of your bust, keeping the tape level all the way around.
Read your size
Pick inches or centimetres, type both numbers, and your estimated size and sister sizes update instantly.
How the size is worked out
The band size comes from rounding your underbust to the nearest even number in inches. The cup is the difference between your bust and underbust: each inch of difference moves you up a cup — 1″ is A, 2″ is B, 3″ is C, 4″ is D, 5″ is DD/E, and so on. A sister size keeps the same cup volume while changing the band by one step up or down, which can fix a band that digs in or rides up. This is for general guidance to help you shop, not medical advice or a substitute for a professional fitting.
Questions
Is anything uploaded?
No. Your measurements stay on your device — the calculation runs entirely in your browser and nothing is sent or stored.
Why does the size feel off in some bras?
Cut, fabric and brand all change the fit. Use the result as a starting point and try the sister sizes if the band or cup isn't quite right.
Can I enter centimetres?
Yes. Switch the unit toggle to centimetres and the tool converts your measurements before working out the US size.