Estimate a US bra size from your underbust (band) and bust measurements.
Uses the traditional US +4/+5 band method — sizing varies by brand and region, so treat this as a starting point.
This calculator uses the traditional US method: round your underbust to the nearest inch, then add 4 if that number is even or 5 if it's odd, to get the band size. It's a long-standing rule of thumb rather than a precise fit measurement.
The cup letter comes from the difference between your bust (fullest point) measurement and the band size, rounded to the nearest inch — each inch of difference steps up one letter (AA, A, B, C, …) in the CUP_LETTERS scale used here.
Sister sizes have the same cup volume as your calculated size but a different band/cup combination — e.g. going down a band and up a cup, or vice versa. They're shown because brands and styles fit differently, so a sister size is often worth trying if your calculated size doesn't fit well.