Bra size calculator
A 75B and an 80A hold almost the same cup volume: going up a band size and down a cup letter keeps it roughly constant. Hence "sister sizes": if the cup fits but the band rides up, going down a band and up a cup keeps the volume while tightening the fit. Most of the support comes from the band, not the straps.
Sizing varies considerably between brands and styles. Treat this as a starting point for trying on rather than a definitive size.
Band size comes from the underbust measurement and cup size from the difference between bust and underbust — 2 cm a letter on the continental scale, an inch a letter in the UK and US. A 75 cm underbust with a 90 cm bust is a 15 cm difference, which this panel reads as 75B, with 80A and 70C as the sister sizes either side of it. The same measurements are a 30DD in UK and US labelling, and that is not a relabelling of the same letter: the two systems measure the cup from different starting points, so they part company by several letters rather than one.
How to measure for a bra
Two different methods are in circulation and they do not agree. The older one, still printed on plenty of charts, tells you to add four or five inches to the underbust measurement before setting the band. It dates from an era of far less elastic fabric and it produces a band several sizes too large, which then needs a cup several letters too small to compensate. This panel uses the modern method: the band is the snug underbust measurement, and nothing is added to it. If a chart you have seen elsewhere gives you a much bigger band and a smaller cup, that is the difference you are looking at.
The consequences of getting it wrong are well documented in one specific setting. Greenbaum and colleagues measured 103 women attending a pre-assessment clinic for breast reduction surgery and found that every one of the 102 included was wearing the wrong size. All but one underestimated their back measurement, by an average of four inches, and overestimated their cup size by an average of three letters. That is a selected group rather than the general population, and the popular statistic derived from studies like it is usually overstated, but the direction of the error is consistent: band too big, cup too small.
Sister sizes exist because cup volume is a function of the difference, not of the letter. A 75B and an 80A enclose almost the same volume, so the letter alone tells you nothing without the band beside it. If the cup fits but the band rides up at the back, going down a band and up a letter keeps the volume while tightening the fit. Most of the support comes from the band, which is why riding up is the single clearest sign that a bra is wrong.
None of this is standardised, which is why the panel prints three rows rather than one. There is no international standard for cup lettering. Below DD the three scales agree; above it they part company, the UK doubling its letters as DD, E, F, FF, G, the US stacking D as DD, DDD and then switching to single letters at G, and the continental scale running straight through E, F, G, H. EU bands round the underbust to the nearest five centimetres while UK and US bands step by two inches, and the letter step itself is a convention, not a measured quantity. Any conversion between systems, including this one, is an approximation over an unstandardised scale.
What people use it for
- Working out a starting size before shopping
- Understanding what a sister size is and when to reach for one
- Converting between EU and UK sizing
- Diagnosing why a bra rides up or gapes
- Checking a size against a chart that uses the older add-four-inches method
- Re-measuring after a change in weight
Questions
Underbust snug and level for the band, bust loose at the fullest point. The difference between them sets the cup letter.
No. That is the older method and it dates from much less stretchy fabric. It puts you in a band several sizes too big, which then needs a smaller cup to compensate, and the result is a bra supported by its straps.
A size holding roughly the same cup volume on a different band: up a band and down a letter, or the reverse. In centimetres the three shown here are 75B, 80A and 70C. Switch the panel to inches and the band steps by two instead, giving 30DD with 32D and 28E.
It is too loose. Go down a band size and up a cup letter, which tightens the fit while keeping the volume.
Only up to D, which is why the panel prints a row for each. Both call the fifth step above A a DD, and American labels often write that one as E. At the sixth the UK says E where the US says DDD, sometimes written F. From the seventh the US runs single letters — G, H, I, J — while the UK keeps doubling: F, FF, G, GG, H, HH.
Not a meaningful one. Cup lettering above D varies between countries and manufacturers, and band conventions differ too, which is why a size is a starting point for trying things on rather than an answer.
The commonly quoted figures come from small studies in selected groups. In one clinic sample of 102 women referred for breast reduction, every single one was in the wrong size, with the back measurement underestimated by four inches on average. That is a strong finding about that group and a weak one about everybody else.
Whenever weight changes noticeably, and roughly annually otherwise. Bands stretch with wear, so an old bra fits differently from a new one in the same size.
Brand variation is large, and the cup step of 2.5 cm is a convention, not a physical constant. Take the calculated size and its two sisters into the fitting room and let the fit decide.