Bra Size Calculator
Convert underbust and bust measurements into a band and cup size.
Reviewed by the ToolNestr Editorial Team — July 2026
How bra sizing works
Bra size is built from two separate measurements. Band size comes from your underbust circumference, rounded to the nearest even number since bands are conventionally sold only in even sizes (30, 32, 34, and so on). Cup size comes from the difference between your bust measurement and that band size — each full inch of difference steps up one cup letter, starting at AA for less than one inch of difference, A for one inch, B for two inches, C for three inches, D for four inches, and continuing upward from there.
This two-part system is why cup size is not a fixed physical size on its own — a D cup on a 30 band covers meaningfully less volume than a D cup on a 38 band, since cup size only measures the difference relative to that specific band, not an absolute size.
Sizing follows the standard band-plus-difference method used across the US and UK lingerie industry, cross-checked against common professional bra-fitting guidance.
Worked example
A measured underbust of 29 inches and a bust of 36 inches.
Real-world context
Studies cited by professional bra fitters have long suggested a large majority of people wear the wrong band or cup size, most often a band that is too large paired with a cup that is too small — the two-measurement method here exists specifically to correct that common self-sizing error, since guessing based on a remembered size from years ago rarely accounts for weight or body changes since the last professional fitting.
Online shopping across US and UK retailers is where the cup-letter difference matters most in practice — a shopper who knows they wear a US DDD needs to look for a UK F on a British site, not a UK DDD, since the two countries' letter systems diverge above D.
Common misconceptions
"A D cup is a specific, fixed size regardless of band." It isn't. Cup size is only meaningful relative to its band — a 30D and a 40D differ enormously in actual volume, even though both are labeled "D."
"Sister sizing means any nearby size will fit the same." Not quite. Sister sizes (like 32C and 34B) have the same cup volume but a different band circumference, so they fit differently around the ribcage even though the cup holds a similar volume.
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Frequently asked questions
How do I measure underbust and bust correctly?
Measure underbust snugly around your ribcage directly under the bust, keeping the tape level. Measure bust around the fullest part of your chest while wearing a non-padded bra, keeping the tape level and not pulled tight. Both should be in inches for this calculator.
Why is band size always an even number in US sizing?
US and UK band sizes are conventionally sold only in even numbers (30, 32, 34, 36...), so the underbust measurement is rounded to the nearest even number rather than used exactly as measured.
Do US and UK cup letters mean the same thing?
Not past D. US and UK sizing agree through A, B, C, and D, but the US then uses DD, DDD, and so on, while the UK uses DD, E, F, FF, G — so a US DDD is roughly a UK F, which causes real confusion when shopping across countries.
Why might my calculated size feel wrong once I try it on?
Bra fit depends heavily on brand, wire shape, and fabric stretch, not just the two measurements used here — this calculator gives a solid starting point for which sizes to try on, not a guaranteed exact fit.