Bra Size Calculator
Enter your underbust (band) and bust measurements in inches to estimate your US bra size.
Last updated: September 5, 2026
Frequently asked questions
How do I measure for a bra size?
Underbust: wrap a tape measure snugly around your ribcage, directly under your bust, keeping it level and not too tight. Bust: wrap the tape around the fullest part of your bust, without compressing it, while wearing a well-fitting (non-padded) bra.
How is cup size calculated?
By the difference between your bust and underbust measurements - each full inch of difference corresponds to one cup letter, starting at AA for less than 1 inch, A for 1 inch, B for 2 inches, and so on.
Why do bra sizes vary so much between brands?
There's no single universal sizing standard - brands use different measuring conventions, cut patterns, and fit philosophies, so the same calculated size can fit differently across brands. Use this calculator as a starting point and expect to fine-tune with try-ons.
What if my calculated size doesn't fit well?
This formula is a widely used estimate, not a substitute for professional fitting - if the band feels loose or the cups gap or spill over, try adjacent sizes (a smaller band with a larger cup, or vice versa, often fits similarly) or get measured in person.
What's an example bra size calculation?
If your underbust measures 30 inches and your bust measures 34 inches, the band size rounds to 32 and the 4-inch bust-to-underbust difference corresponds to a D cup, giving an estimated size of 32D.
Is a 'sister size' the same as my calculated size?
A sister size is a different band/cup combination that has a similar cup volume to your calculated size - for example, 32D and 34C hold roughly the same cup volume on different bands. If your calculated size doesn't fit, its sister size is often worth trying.
Does band size get rounded up or down?
Band size is typically rounded to the nearest even number in US sizing - an underbust measurement of 29 inches rounds to a 30 band, and 31 inches also rounds to 30, since bra bands are only manufactured in even-number increments.