Your bra size is two numbers hiding in plain sight: the band that wraps your ribcage and the cup that holds the difference. Measure your underbust and your bust once, drop them into the calculator below, and you will have your real starting size in about 30 seconds. No tape-measure guesswork, no squeezing into whatever the store had left.
Bra Size Calculator
Two measurements in, your band and cup out.
Please enter both measurements, with the bust larger than the underbust.
How the calculator finds your size
Two measurements do all the work. The first, your underbust, sets the band. Wrap the tape snugly around your ribcage directly under your bust, keep it level, and read the number. The second, your bust, is measured loosely across the fullest part. The gap between those two numbers is your cup: roughly one inch of difference per cup letter.
So a 34-inch underbust with a 37-inch bust lands near a 34C, because three inches of difference is about a C. It sounds almost too simple, and that is the point. Most women wearing the wrong size are not far off; they are one band and one cup away from comfort, and they never had the two numbers side by side.
What to do with your result
Treat the size as your fitting-room anchor, then let the mirror have the final word. Put on the calculated size and run a three-point check: the band sits level across your back without riding up, the straps stay put without carving into your shoulders, and the cups contain your bust smoothly with no bulge at the top and no wrinkling in the fabric. If one thing is off, you usually only need to move one step, up or down a band, up or down a cup.
If you are between sizes or your ribcage measurement came out odd, a wireless, stretch-knit style is the forgiving choice, because it flexes with you instead of locking you into one rigid number. Our Seamless Wireless Everyday Comfort Bra and the best-selling CloudBound Wireless Jelly Bra are both built for exactly that in-between comfort.
US vs UK sizes: same body, different letters
The calculator gives you a US size. If you shop UK brands, the cup letters diverge past a D, because UK sizing adds letters the US system doubles up (a US DDD is often a UK E or F, for example). Same body, different label. Before you buy across systems, run your number through our US, UK and EU bra size conversion chart so you are comparing like for like.
And if you want the full measuring walkthrough with tips for getting the tape right, our complete guide to measuring your bra size at home covers every step.
Bra size calculator FAQ
How accurate is a bra size calculator?
A calculator gets you within one band and one cup of your true fit, which is close enough to walk into a fitting room with confidence. It cannot feel how a specific style sits on your body, so always confirm the size by trying it on and checking the band, straps and cups.
Should I round my band measurement up or down?
Round to the nearest even number. Our calculator does this for you, and if your ribcage measures an odd number it rounds up so the band is not too tight. If the result feels loose, sizing down one band and up one cup (a sister size) often fixes it.
What if my bust and underbust are almost the same?
A small difference simply means a smaller cup, often an AA, A or B. That is common and completely normal. A wireless bralette-style cut tends to be the most comfortable and flattering choice at those sizes.
Why does my calculated size look so different from what I usually wear?
Most people have been wearing a band that is too big and a cup that is too small, because a loose band feels comfortable at first. A snug band that gives you a larger cup letter is doing its job: it supports from underneath so the straps do not have to.
