How this instrument works
Bra sizing has no single governing standards body the way, say, screw threads or shoe widths sometimes do — different brands, fitting guides, and countries have historically used different conventions, and even reputable sources disagree on the details. This calculator implements one specific, modern, widely-taught method: the 'direct measurement' convention, where band size comes straight from your underbust measurement (rounded to the nearest inch, then forced to the nearest even number, since US band sizes only come in even numbers) rather than the older 'add 4 or 5 inches to underbust' method some charts still use.
Cup size then comes from the gap between your bust measurement and that band number: round your bust to the nearest inch, subtract the band size, and the resulting difference in inches maps to a cup letter through the standard US AA-through-R difference table — a 0-inch difference is cup AA, a 4-inch difference is cup D, and so on, one letter per additional inch.
Treat the result as a genuinely useful starting point for trying on bras, not a guaranteed fit. Bra fit depends on band snugness, cup shape, wire length, strap adjustment, and brand-to-brand cut differences that no formula measuring only two numbers can fully capture — the same way a shoe-size formula can't account for every brand's last shape. If the band feels loose or the cups gap or spill, size up or down a step from this starting point rather than assuming the calculator's answer is final.
- Measure Underbust — snugly around your ribcage directly under the bust, keeping the tape parallel to the floor.
- Measure Bust — around the fullest part of the bust, over the nipple line, without the tape pulled tight.
- Enter both measurements in inches into the calculator.
- Read US band size and US cup size — your starting-point estimate.
- Try that size on and adjust up or down a step if the band feels loose, the cups gap, or the wire doesn't sit flat against your ribcage.
Worked example — 32 in underbust, 36 in bust
An underbust measurement of exactly 32 inches (already an even number, so no rounding adjustment needed) with a 36-inch bust measurement gives a band size of 32 and a difference of 36 − 32 = 4 inches, which maps to cup D on the standard difference table — a size 32D.
A slightly different case shows the rounding rules at work: a 31-inch underbust rounds to 31, then bumps up to the nearest even number, 32, for the band. A 33.5-inch bust rounds up (half-up convention) to 34, giving a difference of 34 − 32 = 2 inches — cup B, a size 32B.
Questions
Is this calculator based on an official sizing standard?
No — bra sizing has no single ISO or ANSI-style standard the way some other measurements do; it's a set of trade conventions that brands and fitting guides have developed independently and don't fully agree on. This calculator implements one specific, modern, commonly-taught convention (direct-measurement band sizing) and discloses that plainly rather than presenting it as more universal than it actually is.
Why does the band size get rounded to an even number?
Because US bra bands are conventionally manufactured only in even sizes (30, 32, 34, and so on) — you won't typically find a 31 or 33 band on a store shelf, so the calculator rounds your raw underbust measurement up to the nearest size that's actually sold, matching standard retail sizing.
What's the difference between this method and the 'add 4 or 5 inches' method?
The older method added 4 or 5 inches to your raw underbust measurement before rounding to get the band size, a convention that dates back to when bra construction and fabrics were different. The direct-measurement method used here instead rounds the underbust measurement itself with no added inches, which most modern fitting guides consider more accurate for how bras are actually cut and sized today — but you'll still see both conventions in circulation, which is part of why bra sizing across brands can feel so inconsistent.
Why did my bust measurement round the way it did?
The calculator uses round-half-up rounding — a measurement exactly on a half-inch, like 33.5 inches, rounds up to 34 rather than down to 33 or using a statistical banker's-rounding rule. This keeps the rounding behavior predictable and consistent across every input rather than varying case by case.
The calculator's size doesn't match what I usually wear — what should I do?
That's common and expected — brands cut bras differently enough that a measurement-based formula gives a starting point, not a guaranteed match to any specific brand's labeled size. Try the calculated size on first; if the band rides up in back or feels loose, try a smaller band with a correspondingly larger cup (or vice versa for a tight band), since band and cup sizes trade off against each other within roughly the same actual volume.