About the Bra Size Calculator
This tool estimates your bra size using the traditional US band-and-cup measuring method. Two measurements are all it needs: the snug circumference directly under your bust (underbust) and the circumference around the fullest part of your bust. From those two numbers it derives a band size, a cup letter, and the combined size, such as "34B".
The formulas
- Band size: round your underbust measurement to the nearest whole inch. If that number is even, add 4; if it is odd, add 5. The result is your band size (always one of the standard even US bands, like 30, 32, 34, or 36).
- Cup size: round your bust measurement to the nearest whole inch, then subtract the rounded underbust measurement. Each inch of difference is one cup letter: under 1" = AA, 1" = A, 2" = B, 3" = C, 4" = D, 5" = DD, 6" = DDD, and so on up the alphabet.
- Full size: band size followed immediately by the cup letter, e.g. band 34 with a 2" difference (cup B) gives 34B.
Measurement tips
- Measure in a well-fitting, non-padded bra (or no bra), standing straight, with the tape snug but not compressing your ribcage.
- Keep the tape level and parallel to the floor for both the underbust and bust measurements.
- If you enter centimeters, they are converted to inches (1 in = 2.54 cm) before the formula is applied, since this sizing method is inch-based.
Practical context
This formula is a widely used starting point, not a guarantee — cup and band shape vary by brand and style, and many people fall between two sizes. If a result feels off, try the sister sizes noted in your result (same cup volume, different band snugness), or visit a store for a professional bra fitting.
Frequently Asked Questions
How is bra band size calculated?
Measure snugly around your ribcage directly under the bust and round to the nearest whole inch. If that number is even, add 4 to get your band size; if it is odd, add 5. This traditional rule always produces one of the standard even US band sizes, such as 32, 34, or 36.
How is cup size determined?
Measure around the fullest part of your bust and round to the nearest whole inch, then subtract your rounded underbust measurement from it. Each inch of difference corresponds to one cup letter: less than 1 inch is AA, 1 inch is A, 2 inches is B, 3 inches is C, 4 inches is D, 5 inches is DD, 6 inches is DDD, and so on.
What are sister sizes?
Sister sizes are different band/cup combinations with roughly the same cup volume. Moving up one band size (+2 inches) while dropping one cup letter, or moving down one band size while going up one cup letter, keeps the cup volume similar but changes how snug the band feels — useful if your usual size is out of stock or feels off.
Does this calculator work in centimeters?
Yes. Select centimeters and enter your measurements as usual; they are converted to inches (1 inch = 2.54 cm) before the standard band-and-cup formula is applied, since US and UK bra sizing conventions are inch-based.