Band size: The number in your bra size (e.g., 34 in 34C). It corresponds to your underbust measurement rounded to the nearest even number. The band provides most of the support.
Cup size: The letter (A, B, C, etc.) determined by the difference between your bust and underbust measurements. Each inch of difference generally equals one cup size step. Cup volume is relative to band size — a 34D and 36C hold similar volume.
Sister sizes: Alternative sizes that hold approximately the same cup volume. If your calculated size doesn't feel right, try a sister size — one band up with one cup down, or one band down with one cup up.
Starting point only: Bra sizing varies between brands and styles. Use this result as a starting point and always try on before buying.
How This Calculator Works
You enter your underbust and bust measurements in inches or centimeters. The tool converts to inches if needed, rounds the underbust to the nearest even number for the band size, and subtracts the underbust from the bust to determine cup difference. Each inch of difference maps to a cup letter (0=AA, 1=A, 2=B, 3=C, and so on). It then calculates sister sizes by shifting the band up or down by 2 and adjusting the cup one step in the opposite direction.
Quick Questions
How should I measure myself?
Wear an unpadded bra or no bra. For underbust, wrap the tape snugly around your ribcage just below your breasts, keeping it level. For bust, measure at the fullest point of your chest, keeping the tape level and not too tight. Stand straight with arms relaxed.
What if my size falls between two band sizes?
Try both — if the band feels too tight, go up one size; if it rides up your back, go down. The band should be snug on the loosest hook when new, since it stretches over time. Sister sizes can also help bridge the gap.
Why do I get different sizes at different stores?
Bra sizing is not fully standardized. European, UK, and U.S. sizing systems differ, and even within a system, brands vary. This calculator uses U.S. sizing conventions. Always check the specific brand's size chart.
What are sister sizes and when should I use them?
Sister sizes are adjacent band-cup combinations that hold roughly the same cup volume. For example, 34C, 36B, and 32D are sisters. If one dimension fits but the other doesn't, trying a sister size can solve the problem without a custom fit.