Bra size calculator
Two measurements, and everything they can honestly tell you: your size under both band rules in wide use, in US, UK and European sizing, with the sister sizes to try when the band or the cup is off.
By Alex Seote, Built and maintains Rule Calculator
Conventions checked against the published fitting literature · How we check
Starting size, modern method
32DDD
6 inches of difference over a 32 band. In UK sizing that is 32E; in European sizing, 70F.
The two methods, side by side
| Method | US | UK | EU |
|---|---|---|---|
| Round underbust to the nearest even | 32DDD | 32E | 70F |
| Add four inches (or five) | 36B | 36B | 80B |
The two rules disagree on your measurements, and they usually do. The added-inches rule dates from an era of stiffer, less elastic fabric; most fitters and most specialist retailers now use the rounded underbust. If a shop measured you years ago and the band feels loose, this is the likely reason.
Sister sizes — the same cup, a different band
28H · 30G · 34DD · 36D
Each of these holds roughly the same cup volume as 32DDD. If the band rides up your back, go down a band and up a cup; if it digs in, go up a band and down a cup. The cup letter on its own means nothing without the band it sits on — a 32D and a 38D are not the same size.
There is no standard, and that is the finding
Shoe sizes have standards. Bra sizes do not: no body sets the band rule, the cup grading or the letter sequence, and manufacturers differ from each other on all three. That is why the same person is a different size in two shops on the same afternoon, and it is not a measuring error.
What the research adds is that measurement alone does not close the gap. Studies of women referred for reduction surgery found the great majority in a size that did not fit, and work on fitting criteria found trained assessment outperformed the tape. Treat the size above as the first one to try on, and judge the band and the cup separately once it is on.
Questions people actually ask
- Why do calculators give me different sizes?
- Because they use different band rules. The older method adds four inches to the underbust measurement — five if four lands on an odd number — and dates from a time when band fabric had far less stretch. The modern method rounds the underbust to the nearest even number and leaves it there. On the same body the two can differ by four cup letters, and neither is a standard: they are conventions, and shops disagree about which to use.
- Is a 34D the same cup as a 38D?
- No. A cup letter is a difference from the band, not a volume, so the cup grows with the band it sits on: 34D and 38D share a letter and nothing else. That is why sister sizes exist — 34D, 32DD and 36C hold roughly the same volume on different bands — and why "I am a D" is not a size.
- What is a US DDD in UK sizing?
- An F, usually. Above D the three systems stop agreeing: the US runs D, DD, DDD, then letters; the UK runs D, DD, E, F, FF, G, GG; European sizing runs straight through the alphabet without doubling. The table above converts all three, and it is a lookup rather than arithmetic — which is why converting in your head goes wrong.
- How accurate is a measured size?
- It is a starting point, not an answer. Greenbaum and colleagues found that among women referred for breast reduction, the overwhelming majority were wearing a size that did not fit; McGhee and Steele found that professional fitting by criteria outperformed measurement. Breast shape, tissue firmness, wire width and brand grading all move the size that actually fits, and none of them is in a tape measure.
- How should I measure?
- Underbust snug and level, straight around the ribcage, breathing out. Bust loose across the fullest part, without compressing. Wear an unpadded bra or none, and measure in front of a mirror to keep the tape level at the back — a tape that rides up gives a smaller underbust and a larger cup, which is the single most common measuring error.
- How do I know it fits?
- The band does the work — about 80% of the support — so it should be firm and level, not riding up, on the loosest hook when new. The wire should sit flat against the ribs and behind the breast tissue, not on it. Any spillage at the top or gaping at the edge is a cup problem, and the band and cup have to be judged separately, using sister sizes to move one without the other.