Bra Size Calculator
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| Sister Size (Same Cup Volume) | Band | Cup |
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How the Band-and-Bust Method Works
This calculator uses the standard measurement method used by most lingerie fitters and manufacturers. First, the band size is calculated from your under-bust measurement, rounded to the nearest even number (for measurements that fall on an odd number, the convention is to round up). Then the bust measurement is taken at the fullest point, and the difference between bust and band determines the cup size: each inch of difference steps up one cup letter, starting at A for a 1″ difference (0″ or less is AA). So a 4″ difference is a D cup, a 5″ difference is a DD/E, and so on. Sizing charts vary slightly between brands and countries, so treat the result as a well-informed starting point rather than a guaranteed fit — always try a size on and check that the band sits snug and level and the cups fully contain the breast tissue with no gapping or spillage.
Why "Sister Sizes" Matter
A sister size is a different labeled size that shares roughly the same cup volume as your calculated size, because cup volume is relative to band size, not an absolute measurement. Moving one band size up and one cup letter down (or vice versa) keeps the cup volume about the same while changing how snug the band feels — for example, a 34C and a 36B hold a similar amount of volume. This is why, if your calculated size runs tight in the band or loose in the cup, trying a sister size from the table above is the standard next step before assuming the calculation is wrong.
Related Tools
If you're converting measurements between systems, the conversion calculator can help translate inches to centimeters or vice versa. For sizing outside of lingerie, see the shoe size conversion calculator.
Frequently Asked Questions
How do I measure my band and bust size correctly?
Measure the band snugly around your rib cage directly under your bust, keeping the tape parallel to the floor. Measure the bust at the fullest part of your chest, usually across the nipple line, while wearing a non-padded bra. Both measurements should be taken with the tape snug but not compressing the skin.
Why do two different labeled sizes fit me the same way?
Those are sister sizes. Cup volume is relative to band size, so moving one band size up while dropping one cup letter (or the reverse) keeps roughly the same cup volume but changes how the band feels. If your calculated size is tight in the band, try the sister size with a larger band and smaller cup letter, and vice versa.