A bra size measurement is really two separate measurements working together — not one number, and not a guess. Here's what each one represents.
Underbust circumference
Taken directly beneath the bust, snug against the ribcage. This becomes your band number, and it's the part of the bra that should carry most of the support.
Fullest point circumference
Taken around the fullest part of the chest, resting rather than pulling. On its own it doesn't give a cup size — it needs the band number too.
The difference between the two
Subtract the band number from the bust number, in inches, and that difference maps to a cup letter — a bigger gap means a bigger cup.
cup difference → cup letter (0=AA, 1=A, 2=B, 3=C, 4=D ...)
This is why the same bust measurement can produce a completely different cup size on two different people — it all depends on how that number compares to their individual band size.
Questions
Bra size measurement FAQs
Either order works — most people find the underbust measurement quicker to get consistent, so it's a natural place to start.
An unpadded, well-fitting bra gives the most consistent reading — a padded bra will inflate your bust measurement specifically.
The underlying measurements are universal, but how they're converted into a labeled size differs slightly between US, UK and EU systems.
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