Bra Size Calculator
Calculate your bra size and see your sister sizes from a snug underbust and full bust measurement in inches.
Sister sizes (same cup volume, different fit): 30D, 34B
This follows the US sizing convention. Brands vary, and fit (band snugness and cup shape) beats any calculator, so measure over a non-padded bra and try nearby sizes if the first one isn't quite right.
How it works
Enter your snug underbust measurement (taken right under the bust, tape snug but not tight) and your full bust measurement (taken at the fullest part). The band size comes from rounding your underbust up to the next even number, and the cup size comes from the difference between your bust and that band number, with each inch of difference stepping up one cup letter: AA, A, B, C, D, DD, then DDD.
Worked example: a 31 inch underbust and a 35 inch bust. Thirty-one rounds up to the next even number, 32, so that's the band. The bust-to-band difference is 35 minus 32, which is 3 inches, and 3 inches lands on a C cup. Put together, that's a 32C. The calculator also shows two sister sizes, 30D and 34B, which have roughly the same cup volume in a different band, worth trying if the first size runs tight in the band or loose in the cup.
FAQ
What's a sister size and why would I need one?
A sister size keeps the same cup volume while shifting the band up or down. If your calculated size fits the cup well but the band digs in, try the sister size one band up (and one cup letter down). If the band rides up in the back, try one band down and one cup letter up.
Why does the band size round up to an even number?
Most brands only manufacture bands in even numbers (30, 32, 34, and so on), so odd or fractional underbust measurements get rounded up to the nearest one that's actually sold.
What happens past a DDD cup?
This calculator clamps at DDD, since letter sizing conventions fork in different directions above that point (some brands use E, F, G; others keep stacking D's). If your difference lands beyond DDD, a specialist fitter or a brand's own extended size chart will serve you better than a generic letter here.
Do I need to be fitted in person to trust this?
A calculator gets you a strong starting point, but the band snugness and how the cup sits (no gaping, no spilling, no underwire on breast tissue) matter more than the number on the tag. Try the calculated size and both sister sizes and pick whichever actually fits best on your body.
For more on fit and finding what actually works for your shape, see how to dress for your body type, how to build a capsule wardrobe that works, and how to care for delicate fabrics, since most bras need the same gentle hand-washing.