A bra size calculator helps determine the correct bra size by measuring the band and bust. First, measure around the ribcage, just under the bust, to get the band size. If the number is odd, round up to the nearest even number. Next, measure around the fullest part of the bust for the bust measurement. Subtract the band measurement from the bust measurement to find the cup size difference. Each inch of difference corresponds to a cup letter: 1 = A, 2 = B, 3 = C, and so on. Combine the band size and cup letter to get the bra size (e.g., 34B). Using the right size improves comfort, support, and posture. Regular checks are recommended as body shapes change over time.