To find bra size measure around your bra directly under your bust, breathing out, making this measurement be as small as possible. . If the measurement is even, add 4" if odd, add 5" . Standing straight, with your arms down, measure the fullest part of your bust and do without pulling. Round off all fractional measures to the nearest whole number. Subtract your band measurement from your cup measurement Usually, for each inch in difference, the cup goes up by one size.