Bra Size Calculator

Your band size is your snug underbust measurement rounded to the nearest whole inch and then to an even number, since bands are sold in even sizes. If it lands on an odd number, the calculator takes the band below, because a firmer band is the more common fix, and shows the sister size up in case that feels too tight.

Your cup size is the difference between your bust measurement and that band, at one cup letter per inch. Centimeters are converted to inches first. The letter is given in US sizing (DD, DDD/F, G, H) and UK sizing (DD, E, F, FF), along with the two sister sizes: one band down and one cup up, and one band up and one cup down, which hold roughly the same cup volume.

Measure in a plain, unpadded bra or none, standing straight in front of a mirror so you can check the tape stays level across your back. For the underbust, pull the tape snug and breathe out. For the bust, let the tape rest around the fullest part without compressing anything. If your bust is soft or full, take the bust measurement a second time leaning forward at 90 degrees and use the larger number. Enter the numbers exactly as you read them; the calculator does the rounding.

A tape measure is the most reliable way to find a starting size, but it is still a starting size. Brands cut their bands and cups differently, and a style that suits one breast shape can fit badly on another at the same size, so expect to try the size this gives you and one of its sister sizes.

The cup formula is least reliable at the ends of the range. On a soft or full bust the standing measurement can come up short, and many fitters suggest also measuring leaning forward at 90 degrees and using the larger number. On a small or shallow bust, a cup either side can fit equally well. Once a bra is on, three rules beat any formula: if the band rides up your back, go down a band and up a cup; if the cups gape, go down a cup; if the wire sits on breast tissue, go up a cup.

Cup letters date from the early 1930s, when manufacturers started selling bras by letter instead of a loose small, medium and large. The letters were never standardized, which is why US and UK labels part ways after DD: a US DDD is a UK E, and a US G is a UK F.

The other thing every chart should tell you, and few do, is that band and cup are not independent. A cup letter only describes a volume relative to a band, so a 34C holds about the same volume as a 32D and a 36B. Those are sister sizes, and they are why this calculator gives you three sizes rather than one. When the cups fit but the band does not, the sister size is the fix, not a new cup letter.

The old plus-four method is worth knowing about only so you can recognise it. It adds four or five inches to the underbust before working out the cup, which pushes the band up a size or two and the cup down to match: someone measuring 30 under and 35 over gets told 34A instead of 30DD. If a calculator elsewhere gave you a very different answer, that is usually why.