Bra Size Calculator

How bra size is actually calculated

A bra size is two numbers wearing one label. The band is a measurement of your ribcage; the cup letter is not a measurement at all, but the difference between your bust and that band. Because the letter is a difference, it only means anything next to a band number. A D cup on a 30 band and a D cup on a 40 band are two very different garments, which is why "I'm a D" tells a fitter almost nothing on its own.

The calculator takes your snug underbust measurement, rounds it to the nearest even inch (bands are made in even sizes only, so an odd number rounds up), then subtracts that band from your loose bust measurement. Each inch of difference is one cup letter.

Worked example: 31 in underbust, 36 in bust

Round 31 up to 32 — that is the band. Then 36 − 32 = 4 inches of difference, and four inches is a D cup in US labelling. The result is 32D. In UK labelling that is also 32D, because the two systems agree up to and including D. In European labelling the band converts to centimetres and rounds to the nearest 5: 32 × 2.54 = 81.3 cm, which rounds to 80, giving 80D.

Change one input and watch how much moves. If the same person measured 33 in under the bust, the band would round to 34, the difference would drop to 2 inches, and the size becomes 34B — same body, two letters apart, because the band anchors everything. That sensitivity is exactly why the underbust measurement has to be pulled snug and level, and why a badly taken tape reading is the single biggest source of a wrong answer.

Where US and UK labels split

Below a D the three systems agree. Above it they drift, and that drift is why an American shopper ordering from a British brand — or the reverse — so often gets a cup that is one or two steps off. Read the size chart on the site you are buying from, not the label in the bra you already own.

Sister sizes: the trick that saves a purchase

Cup volume is tied to the band. Shrink the band by two inches and the same cup letter holds less, so you go up a letter to keep the volume. Grow the band by two inches and you go down a letter. That is a sister size: 30DD, 32D and 34C all hold roughly the same amount of breast tissue.

Use it as a diagnostic. If the cups fit but the band rides up your back, the band is too big — drop to 30DD. If the band feels comfortable on the loosest hook but the cups gape at the top, you may be in the right band with too much cup; try 34C. If the wires sit on breast tissue instead of on the ribcage, that is a cup problem, not a band problem, and no sister size will fix it — you need more cup at the same band.

A quick bad-fit checklist

Fasten on the loosest hook when the bra is new; the tightest hooks are there for when elastic relaxes over months of wear. Then check four things in the mirror. The band should sit level all the way round and you should be able to pull it about an inch off your back, no further. The wires should encircle the whole breast and rest on the ribcage, not on tissue. The cups should be smooth, with no overspill at the top or underarm and no wrinkling. And the centre gore between the cups should lie flat against your sternum — a gore that floats is a reliable sign the cups are too small.

Limitations worth knowing

This calculator gives one number from two measurements, and no formula can see breast shape, root width, tissue density or how a specific style is cut. Two people at 32D can need different bras entirely: one full on top, one full on the bottom, one with a wide root that needs a wider wire. Size also moves with the menstrual cycle, weight change, pregnancy, breastfeeding and hormonal medication, so remeasure once or twice a year rather than trusting a fitting from three years ago. Treat the result as the size to bring into the fitting room first, then let the mirror settle the argument.